hive-forge: timeline verb — issue/PR audit trail (closes #783)

last unstarted piece of the original #694 epic. agents kept falling
back to curl for 'who closed this?' / 'when was this labelled?'
archaeology because view + comments only surface the body + comments,
not the structured timeline events (label adds, assignments, closes,
reopens, pushes, commit refs, review submissions, milestone changes).

separate verb rather than view --timeline because:
- composes naturally with view <n> / comments <n>
- keeps existing verb output shapes stable (no script breakage)
- argus on #770 already noted view's output is busy

human-readable by default ('**actor @ ts**: <summary>'), --json for
raw piping. unknown event types fall through to a '[<type>]'
placeholder so a forge schema bump doesn't panic the verb.

--tail N is a follow-up: timeline endpoint doesn't expose a total-count
field so the count-then-page strategy from #770 doesn't apply
directly.

6 tests cover comment / label add/remove / close / unknown-type
placeholder / missing-user fallback.

hive-forge/src/verbs/mod.rs

@@ -23,6 +23,7 @@ pub mod pr;
 pub mod pr_create;
 pub mod pr_reviews;
 pub mod subscription;
+pub mod timeline;
 pub mod tree_sha;
 pub mod view;