Timeline: prevent message rows from overlapping (CL-6784)

[TheGreatAxios]

What changed

• prevent timeline message groups and body children from shrinking into adjacent rows
• keep failed and cancelled turn strips within the message gutter
• preserve a compact single-row failed-turn layout while allowing details to wrap

Verification

• bun run check (passed, exit 0)
• DB-gated test skipped because DATABASE_URL is unavailable

[TheGreatAxios]

CSS-only fix: flex-column timeline items were shrinking below content height on overflow, letting the next row render on top of the previous one. Adding flex-shrink: 0 / min-width: 0 on the flex children and switching the failed/cancelled-turn strips to calc(100% - 2.9rem) against the existing gutter addresses the actual containment mechanism rather than pasting on a fixed height or !important. No !important anywhere in the diff. The 2.9rem gutter is a pre-existing convention already used at 13+ other sites in this file, not a new magic number. Scope is correct per AGENTS.md — packages/chat-ui is workbench-specific timeline composition, not a react-ui primitive, so this doesn't belong upstream.

Must fix

• None blocking.

Consider

• .chat-hover-toolbar's top moved from -0.4rem to -0.25rem with no comment tying it to CL-6784 — reads as an uncommented drive-by tweak riding along with the overlap fix. Either fold it into the CL-6784 comment's rationale or split it out.
• min-height: 1.75rem on .chat-turn-failed / .chat-turn-cancelled is a new magic number with no comment on why 1.75rem specifically (vs. deriving from the retry button's rendered height). Low severity since the file has no spacing-token system to defer to, but worth a one-line note.
• No screenshot or before/after evidence attached, and this is exactly the kind of visual-only regression AGENTS.md calls out CI green as insufficient proof for ("API-only e2e suites kept passing while the real UI broke"). Worth a quick dense-timeline screenshot (many consecutive short messages, a failed turn, a cancelled turn) before merge, given the increased gap (0.1rem → 0.35rem group gap, new 0.2rem body-child gap) changes vertical density everywhere, not just the overlap case.

Verified

• bun run check, lint, typecheck, structural, e2e, isolation, db-suites, build-test all pass per gh pr checks 577.
• Diff is confined to packages/chat-ui/src/styles.css, 64/-8 lines, matching the PR's stated scope.
• Confirmed via git show origin/main:packages/chat-ui/src/styles.css that .chat-message-group previously had no base display/layout rule and .chat-message-row had only position: relative — the flex-shrink/min-width additions are new, not a revert of something removed.