feat(tui): add interactive session search

[albertdbio]

Issue for this PR

Closes #4714

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds interactive incremental search over the current session transcript — readline/less-style rather than a modal prompt.

Press ctrl+r and a search bar attaches to the top of the prompt input:

 │  …transcript scrolls live to the nearest match as you type…       │
 │                                                                    │
 ┃  markdown_hash                            3/17  ↵ keep · esc back  ← search bar
 ┃  > prompt input (untouched)                                        ← prompt

Behavior

• Typing live-jumps the viewport to the nearest match at-or-above your position (reverse-i-search semantics); refining the query stays on the current match when it still matches
• ctrl+r/up = older match, ctrl+s/down = newer, wraparound with a 3/17 counter; bar turns red on zero matches
• esc/ctrl+c cancels and restores the exact scroll position (sticky-bottom aware); enter accepts and keeps position; ctrl+r on an empty bar recalls the last query
• Smartcase (any uppercase in the query = case-sensitive), literal matching, unicode-safe offsets
• Matches are occurrence-level (a term appearing 5x in one long reply is 5 distinct stops) with soft-wrap-aware row estimation so matches center on screen
• Also available as /search, /find, and command palette entries; search state resets on session switch

In-place highlighting (including inside markdown)

Markdown prose and code blocks render through CodeRenderable, so this uses its public onHighlight hook to append match spans to the tree-sitter highlights — no opentui changes needed. Non-active matches render in a neutral tone, the active match in warning (bold), via two new syntax scopes (search.match, search.match.active). User messages get the same treatment with text spans. Highlighting only engages when total hits <= 200 so broad one-letter queries don't re-highlight every part per keystroke.

Keybind note (deliberate, happy to change)

messages_search defaults to ctrl+r, and session_rename moves to none (still available in the command palette and remappable). Rationale: reverse-search muscle memory is strong, and rename felt like a lower-frequency action to hold a prime binding. If you'd rather keep rename on ctrl+r, I can default search to <leader>/ or anything else — one-line change.

Related prior art: #34297 takes a dialog-based approach with message-level granularity; this PR went inline/incremental after reading the feedback in #4714 about UI placement.

How did you verify your code works?

• bun test from packages/tui (25 new unit tests over the pure search core: unit collection mirroring rendered content, occurrence offsets/lines, smartcase, regex-literal escaping, astral unicode offsets, wraparound, viewport-relative initial-match selection, highlight tuples, soft-wrap row estimation) — 217 pass
• bun typecheck from packages/tui — clean
• Extensive manual TUI testing in a real terminal: open, live jump, cycle+wrap, esc restore, ctrl+c close, empty-bar recall, 0/0 state, smartcase, in-markdown highlight rendering (prose + code blocks), active-match cycling, streaming sessions, session switching, light/dark theme input visibility

Screenshots / recordings

Happy to attach a recording if useful.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found related PR:

• PR feat(tui): add session search #34297: feat(tui): add session search

  • feat(tui): add session search #34297

  This PR is explicitly mentioned in the current PR description as prior art that "takes a dialog-based approach with message-level granularity." The current PR (36526) is a different implementation approach—inline/incremental search rather than modal—based on feedback in issue [FEATURE]: TUI - Search for and find string in session buffer #4714.

These are related but not duplicates; they represent different design choices for the same underlying feature (session search).