refactor(git): model review-url resolution with typed entries

Addresses the review on #45.

Move `review-url` out of `git.rs` into `src/commands/git/review_url.rs`, so
`git.rs` holds the command variants and local VCS work again rather than
GraphQL queries, feed pagination, and API decoding. `git.rs` returns to
roughly its pre-feature size.

Replace the raw-`Value` merge pipeline with typed deserialization and a
serializable `ReviewEntry`. Sources merge into one map keyed by GitHub pull
request URL, with the agent-session fallback inserted first so a notification
result replaces it — precedence is now the merge's structure rather than a
convention about ordering. Missing fields are `Option`s instead of silent
nulls, and a pull request Linear returns without a URL yields no entry rather
than one with a null identity.

Resolution is modelled as resolved plus unresolved pull requests. Previously
an issue with two attached pull requests where only one resolved printed that
one URL and exited 0, saying nothing about the other. Unresolved pull requests
are now part of the output contract: `-o json` returns
`{"resolved": [...], "unresolved": [...]}` and the plain-text form names them
on stderr. The command still fails only when it resolved nothing.

Drop the private cursor state machine in favour of `paginate_until`, a
short-circuiting paginator alongside `paginate_nodes` in `pagination.rs`. It
follows the canonical cursor rules — including stopping when a connection
claims `hasNextPage` without returning an `endCursor`, which the private loop
would have answered by rereading the first page until its five-page cap. It
stops as soon as every wanted pull request is found, so the common
resolved-on-the-first-page case still costs one request rather than five.

Also drop a trailing blank line in `initiatives.rs` that failed
`cargo fmt --check` and so kept Clippy from running.

Author: oliviasculley

README.md

@@ -367,8 +367,13 @@ linear-cli g review-url LIN-123                  # Linear review URL for the iss
 the one public place a pull request is paired with its review page — and falls
 back to the pull requests linked to the issue's agent sessions. A pull request
 that has produced neither (a brand-new PR with no CI result, comment, or review
-activity yet) has nothing to resolve, and the command says so instead of guessing
-a URL.
+activity yet) has nothing to resolve, and the command reports it instead of
+guessing a URL.
+
+Unresolved pull requests are part of the output, not a silent omission: `-o json`
+returns `{"resolved": [...], "unresolved": [...]}`, and the plain-text form prints
+the review URLs on stdout while naming any unresolved pull request on stderr. The
+command fails only when it resolved nothing at all.
 
 ### Import / Export