Add branch-addressed merge with --branch

docs/src/content/docs/merge.md

@@ -1,14 +1,14 @@
 ---
 title: "wt merge"
-description: "Merge current branch into the target branch. Squash & rebase, fast-forward the target branch, remove the worktree."
+description: "Merge a branch into the target branch. Squash & rebase, fast-forward the target branch, remove the worktree."
 sidebar:
   order: 12
 ---
 <!-- ⚠️ AUTO-GENERATED from `wt merge --help-page` — edit src/cli/mod.rs to update -->
 
-Merge current branch into the target branch. Squash & rebase, fast-forward the target branch, remove the worktree.
+Merge a branch into the target branch. Squash & rebase, fast-forward the target branch, remove the worktree.
 
-Unlike `git merge`, this merges the current branch into the target branch — not the target into current. Similar to clicking "Merge pull request" on GitHub, but locally. The target defaults to the default branch.
+Unlike `git merge`, this merges the source branch into the target branch — not the target into current. Similar to clicking "Merge pull request" on GitHub, but locally. The source defaults to the current branch; the target defaults to the default branch.
 
 <figure class="demo">
 <picture>
@@ -43,6 +43,15 @@ Merge to a different branch:
 $ wt merge develop
 ```
 
+Merge another worktree's branch without leaving the current worktree:
+
+```console
+$ wt merge --branch feature/auth
+$ wt merge develop --branch feature/auth
+```
+
+The source branch must have a checked-out worktree. Commit, squash, rebase, and hooks use that worktree and its project configuration. Cleanup removes the source worktree; the invoking worktree stays in place. A rebase conflict is left in the source worktree to resolve or abort.
+
 Keep the worktree after merging:
 
 ```console
@@ -83,7 +92,7 @@ $ wt merge --no-commit --no-rebase
 4. **Pre-merge hooks** — Hooks run after rebase, before merge. Failures abort. See [`wt hook`](/hook/).
 5. **Merge** — Fast-forward merge to the target branch ([`wt step push`](/step/#wt-step-push)). With `--no-ff`, a merge commit is created instead — semi-linear history after the default rebase, while explicit `--no-rebase` preserves the graph produced by earlier steps before adding the merge commit. Non-fast-forward merges are rejected.
 6. **Pre-remove hooks** — Hooks run before removing worktree. Failures abort.
-7. **Cleanup** — Removes the worktree and branch. Use `--no-remove` to keep the worktree. When already on the target branch, in the primary worktree, or locked, the worktree is preserved.
+7. **Cleanup** — Removes the worktree and branch. Use `--no-remove` to keep the worktree. When already on the target branch, in the primary worktree, locked, or containing another registered worktree, the worktree is preserved.
 8. **Post-remove + post-merge hooks** — Run in background after cleanup.
 
 Use `--no-commit` to skip committing uncommitted changes and squashing; rebase still runs by default and can rewrite commits unless `--no-rebase` is passed. Combining both flags preserves the exact source graph and requires the target to be its ancestor. Useful after preparing commits manually with `wt step commit`. Requires a clean working tree.
@@ -114,7 +123,7 @@ lint = "cargo clippy"
 ## Command reference
 
 ```text wt-command-reference
-wt merge - Merge current branch into the target branch
+wt merge - Merge a branch into the target branch
 
 Squash & rebase, fast-forward the target branch, remove the worktree.
 
@@ -127,6 +136,9 @@ Arguments:
           Defaults to default branch.
 
 Options:
+  -b, --branch <BRANCH>
+          Branch to merge from (defaults to current worktree)
+
       --no-squash
           Skip commit squashing
 

src/cli/mod.rs

@@ -556,6 +556,10 @@ pub(crate) struct RemoveArgs {
 
 #[derive(Args)]
 pub(crate) struct MergeArgs {
+    /// Branch to merge from (defaults to current worktree)
+    #[arg(short, long, add = crate::completion::worktree_only_completer(), value_parser = crate::cli::non_empty_branch)]
+    pub(crate) branch: Option<String>,
+
     /// Target branch
     ///
     /// Defaults to default branch.
@@ -1429,11 +1433,11 @@ Detached worktrees have no branch name. Pass the worktree path instead: `wt remo
 "#)]
     Remove(RemoveArgs),
 
-    /// Merge current branch into the target branch
+    /// Merge a branch into the target branch
     ///
     /// Squash & rebase, fast-forward the target branch, remove the worktree.
     #[command(
-        after_long_help = r#"Unlike `git merge`, this merges the current branch into the target branch — not the target into current. Similar to clicking "Merge pull request" on GitHub, but locally. The target defaults to the default branch.
+        after_long_help = r#"Unlike `git merge`, this merges the source branch into the target branch — not the target into current. Similar to clicking "Merge pull request" on GitHub, but locally. The source defaults to the current branch; the target defaults to the default branch.
 
 <!-- demo: wt-merge.gif 1600x900 | Creating a worktree, committing in it, and merging it away -->
 ## Examples
@@ -1462,6 +1466,15 @@ Merge to a different branch:
 $ wt merge develop
 ```
 
+Merge another worktree's branch without leaving the current worktree:
+
+```console
+$ wt merge --branch feature/auth
+$ wt merge develop --branch feature/auth
+```
+
+The source branch must have a checked-out worktree. Commit, squash, rebase, and hooks use that worktree and its project configuration. Cleanup removes the source worktree; the invoking worktree stays in place. A rebase conflict is left in the source worktree to resolve or abort.
+
 Keep the worktree after merging:
 
 ```console
@@ -1502,7 +1515,7 @@ $ wt merge --no-commit --no-rebase
 4. **Pre-merge hooks** — Hooks run after rebase, before merge. Failures abort. See [`wt hook`](/hook/).
 5. **Merge** — Fast-forward merge to the target branch ([`wt step push`](/step/#wt-step-push)). With `--no-ff`, a merge commit is created instead — semi-linear history after the default rebase, while explicit `--no-rebase` preserves the graph produced by earlier steps before adding the merge commit. Non-fast-forward merges are rejected.
 6. **Pre-remove hooks** — Hooks run before removing worktree. Failures abort.
-7. **Cleanup** — Removes the worktree and branch. Use `--no-remove` to keep the worktree. When already on the target branch, in the primary worktree, or locked, the worktree is preserved.
+7. **Cleanup** — Removes the worktree and branch. Use `--no-remove` to keep the worktree. When already on the target branch, in the primary worktree, locked, or containing another registered worktree, the worktree is preserved.
 8. **Post-remove + post-merge hooks** — Run in background after cleanup.
 
 Use `--no-commit` to skip committing uncommitted changes and squashing; rebase still runs by default and can rewrite commits unless `--no-rebase` is passed. Combining both flags preserves the exact source graph and requires the target to be its ancestor. Useful after preparing commits manually with `wt step commit`. Requires a clean working tree.

src/commands/hooks.rs

@@ -57,8 +57,8 @@
 //! set. (Aliases get the property structurally instead: the body is frozen
 //! into `AliasEntry` before the gate, like `ApprovedHookPlan`.)
 //!
-//! `ctx.repo` is the invoking worktree — except `wt step commit --branch <b>`
-//! and `wt -C <path>` re-root the whole command (the commit, its hooks, and
+//! `ctx.repo` is the invoking worktree — except `wt step commit --branch <b>`,
+//! `wt merge --branch <b>`, and `wt -C <path>` re-root the whole command (the commit, its hooks, and
 //! `ctx.repo` are all `<b>`), so "the invoking worktree" follows them.
 //!
 //! A present-but-malformed config aborts the operation rather than silently