feat!: consolidate the timeline into Overview

[osolmaz]

Summary

The Activity tab showed a subset of what Overview already shows, and the [details] button existed mainly to jump into it.
Both are removed: Overview is now the single timeline, and everything the pair did is covered by mechanisms that already exist.
Reading a long entry is [more]/[less] expansion, comment permalinks appear on expanded entries, and clicking a same-resource comment link scrolls to that entry on Overview instead of switching tabs.
This also deletes a second activity row builder that had to be kept in sync with Overview's by hand.

What Changed

One timeline, one row builder, five tabs.

• Tab::Activity is gone: PRs show Overview/Commits/Checks/Files/Links, issues show Overview/Links. Number keys, arrows, and vim keys follow the shorter lists; help and README updated. Old sessions or --tab activity values fall back to Overview.
• The Activity-only row builder (activity_rows) and the [details] row are deleted. Overview's conversation entries now carry the focus anchor, so focus_activity_url (same-resource comment links) expands and scrolls to the entry in place.
• Offline refresh clicks are now pure no-ops: the deferred-refresh flag no longer stays set, which previously fired a fixture reload that overwrote the "refresh skipped" status a frame later (the mouse-smoke suite raced against this).
• Capture suite reworked: activity frames and the details mouse flow removed from both scenario roots, tab markers updated, all captures re-recorded.

Testing

Everything regenerated and green locally with exit codes checked individually (no pipelines masking failures).

• cargo fmt --check, cargo check --all-targets, cargo clippy --all-targets --all-features -- -D warnings
• cargo test: 689 tests pass; tab cycling/numbering, focus-to-Overview, and details-absence assertions updated or added
• cargo llvm-cov --fail-under-lines 85: passes
• cargo mutants --in-diff: 1 survivor (the commits parse arm), killed with an exhaustive tab-name parse test; CI runs the gate again
• Both capture roots and both mouse-smoke suites re-recorded and validating; manifests re-stamped
• Codex review: one P2 (stale parser error text) and one P3 (help promising 1-6) — both fixed

Risks

This intentionally removes UI; the risk is workflow breakage for anyone relying on the tab.

• The discussion-only filter view is gone. Overview interleaves commits and the description with comments; heavy PR conversations are read there now.
• Session snapshots and --tab values naming activity silently open Overview rather than erroring.
• [details]'s browser-open fallback for odd URLs is gone; permalinks remain reachable on expanded entries.

[osolmaz]

Final report: merged as c3e5618. Overview is the single timeline; Activity tab, [details] rows, and the duplicate row builder are gone. Comment-anchor links scroll to the entry in place. Gates: 689 tests, coverage, slophammer, both capture suites re-recorded, in-diff mutants 0 missed, Codex review clean after the parser-error and 1-5 help fixes. One deliberate skip: no permalink affordance on short collapsed entries (expansion shows the url row) — any always-visible replacement would reintroduce the clutter this PR removes.