feat: per-thread default working directories

[ytaskiran]

Each thread can carry an optional default working directory. Sessions launched from that thread start there. Threads without one start in the home directory.

How it works

Press c on a thread to open the directory picker. The top row is the directory you are browsing, and the cursor starts there:

 › ~/projects/                    ← Enter here accepts this directory
──────────────────────────────
   agent-harnesses/
   archive/

• Enter on the top row accepts the directory you are browsing
• Enter on a subdirectory descends into it, so Enter never records a directory you did not navigate to
• Tab completes into the first match, so typing a segment and tabbing walks downward like a shell
• Backspace deletes a filter character, or goes up a level when the filter is empty
• Esc cancels

Typing filters the list; the filter is dimmed because Enter takes the directory, not the typed text. To point a thread back at home, Backspace up to ~ and press Enter.

Creating a thread opens the picker right after you name it. Esc there leaves a valid thread with no directory. The tree shows each thread's directory as a dim suffix, brighter on the selected row.

Notes for review

• Behavior change: threads with no directory set now open in ~ rather than inheriting the directory tws was launched from. Launch behavior no longer depends on where you started the app.
• A stale directory never blocks a launch. If a thread's directory is deleted, the session still starts, in ~, and tmux shows a warning. Inside tmux this goes through display-message, because switch-client ends the app before another frame is drawn.
• Paths are stored absolute; the ~ form is display-only.
• Thread.working_dir uses #[serde(default)], so existing state.json files load unchanged, and an older binary ignores the field.
• All picker logic lives in core/workdir.rs with no ratatui dependency, which is what makes it unit-testable. app.rs grows one Mode variant and a handler that delegates every keystroke.
• centered_rect moved from finder_modal.rs to components/mod.rs so both modals share one copy.
• New [keys.dir_picker] config section; every binding is rebindable.

Verification

• 198 tests pass; cargo fmt --all --check and cargo clippy --all-targets -- -D warnings clean
• 33 unit tests cover the picker: listing rules, hidden-directory handling, filtering, cursor model, navigation, selection, and the launch-directory resolution table
• Driven manually in an isolated tmux sandbox: picker navigation, Enter/Tab semantics on each row, persistence across restart, a launched session's pwd, the home fallback, and the stale-directory path both falling back and surfacing its warning

Review round

Eight findings from a review pass were verified against the code and all seven actionable ones fixed in f5811e9: the shorten_home fake-~ bug when home is unresolvable, new_session's discarded failure, the picker's cwd fallback, the filter reading as a path segment, "no subdirectories" vs "no matches", the stale Enter hint and missing Backspace hint, and a vestigial Option on the setter. One was deliberately deferred and recorded: previewing which entry Tab will jump to under a substring filter.

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