docs(list): drop the stale-branch and dirty-tree skips from JSON help

[worktrunk-bot]

wt list --help (and the generated list.md pages, skill mirrors, and list-v2.json schema) explains ·, JSON null, and default_branch.integration in ways the collector no longer matches. This corrects three claims. No behavior changes.

• "branch too stale" for · and null. Cache remaining expensive tasks, unify wt list and picker #2098 removed the stale-branch skip (stale_branches), and nothing in src/commands/list/collect/ skips tasks by branch age now. The remaining sources of null are a timeout, a skipped probe (column plan or unborn branch), or a failed forge fetch.
• integration "null when a dirty tree skipped the checks". Dirty trees don't skip the probes. They run on the committed HEAD either way, as json_default_branch says and test_integration_positive_match_ignores_cleanliness pins with a dirty trees_match result. A dirty integrated branch reports its reason, not null.
• "every reason renders as the same ⊂". "same_commit" renders as _ (or – with uncommitted changes), per IntegrationReason::SameCommit and the Default-branch symbol table in the same help page. ⊂ also needs a clean tree.

The same commit updates the JsonDefaultBranch::integration docstring, which generates the published schema description, and four internal comments in collect/execution.rs that still listed the stale-branch skip.

No test is added because this changes only prose. Help snapshots and doc mirrors are regenerated, and test_docs_are_in_sync passes.

[worktrunk-bot]

The status_symbols.rs module doc still has a Gate 3 exception that reads "Exception — stale branches and --skip-tasks:". This PR removes that same stale-branch claim from collect/execution.rs, but this copy sits outside the diff. --skip-tasks doesn't exist as a flag anymore either (no other match under src/). I'm pushing a commit that rewords the exception to cover tasks the column plan skips, which is what still seeds conservative defaults.