feat(tui): keyboard selection, path/preview toggles for session browser

[alecuba16]

Summary

Split A of #387. Adds keyboard selection and display toggles to the session browser overlay.

Note: This PR supersedes the session-browser-toggles portion of #387. The keyboard text selection + copy features remain in a separate split B PR to keep scope tight.

Changes

Keyboard routing through keybindings.rs

• New KeyContext::SessionBrowser variant with configurable default bindings:
  • ↑/↓ or j/k navigate (prevSession/nextSession)
  • PgDn/PgUp page navigation (nextSessionPage/prevSessionPage)
  • Enter resume session (resumeSession)
  • r rename session (renameSession)
  • a toggle paths (toggleSessionPaths)
  • p toggle preview (toggleSessionPreview)
  • Esc close (closeSessionBrowser)
• All keys flow through the configurable keybinding system in crates/core/src/keybindings.rs — no inline hardcoded key checks.

Session browser rendering

• SessionEntry gains a working_dir field, shown under each row when paths are toggled on.
• render_session_browser rewritten with a fixed layout:
  • Windowed list — list_window helper keeps the selection centered in the visible window. No more scrolling past the viewport.
  • Preview panel — optional panel under the list showing the selected session's title, stats, and working directory. Toggled with p.
  • Hint bar — always visible at the bottom, shows current keybindings.

Toggles

• show_paths (toggle with a): shows the working-directory path of each listed session. Does not reveal sessions from other projects — only the sessions already listed.
• show_preview (toggle with p): shows a preview panel of the selected session.

Fix #382

The session browser opened empty because the session-loading code lived in dead App::run code that was never called. The load now happens in run_interactive via a session_list_tx/session_list_rx channel: when the browser opens, an async list_sessions task spawns and drains entries into the browser.

Session resume from browser

session_resume_pending drives resume from the browser, using the same swap logic as the /resume command: load messages, reset file history and turn bookkeeping, update working_dir.

Tests

• 5 session_browser unit tests: toggle defaults, toggles flip flags, list_window windowing, render with paths+preview, render long list walks selection.
• 2 TUI keybinding routing tests: toggles route through keybindings (not inline), rename stays inline (Esc cancels back to Browse).
• All 22 session_browser tests pass. The 2 new TUI tests pass. Clippy clean.
• Pre-existing failure settings_screen::tests::all_entries_returns_expected_settings (expects ≤20 settings, gets 21) is unrelated — it fails on main and this PR does not touch settings.

CRLF preservation

session_browser.rs, keybindings.rs, and docs/commands.md are CRLF files. All additions use \r\n. No LF churn on untouched lines.