feat: show session chat history

[shawnsw]

Summary

Show recent user/assistant chat history for the selected session.

• Collect bounded chat tails from Claude and Codex transcripts
• Exclude tool results and tool inputs
• Sanitize terminal control and bidi characters before rendering
• Keep timeline and file-audit views available behind toggles

Verification

• cargo test chat
• cargo test
• cargo clippy -- -D warnings

[graykode]

lgtm — security clean and actually defenses-positive: the new sanitize_terminal_text strips C0/C1 controls + bidi override/isolate marks before transcript text hits the TUI, and chat lines also pass through redact_secrets. tests cover both (BEL/RLO removal and sk-proj-secret → [REDACTED]).

pushed one merge commit (9884794) to resolve conflicts with main:

• #112 (is_synthetic_user_msg) replaced this PR's is_tool_result_user_msg. resolved by reusing the broader is_synthetic_user_msg for the chat-tail gating too — slash-command/bash/meta-caveat user lines are now also kept out of chat history, which matches the spirit of this PR (only show real user/assistant turns).
• #113 (codex token comment) was a trivial co-located addition, kept both sides.

after merge, cargo build clean and all 128 tests pass on macOS, including this PR's three new tests and #112's slash-command thinking-window test.

one nit, non-blocking: clean_chat_text is duplicated between collector/claude.rs and collector/codex.rs and the truncation tail differs — claude uses truncate(…, max) which appends …, codex uses chars().take(max).collect() (no marker). worth unifying in a follow-up so users don't see one panel ellipsize and the other hard-cut.

(unrelated: cargo clippy --all-targets -- -D warnings still fails on src/ui/sessions.rs items-after-test-module, but that's pre-existing on main, not this PR.)