feat(app): directory autocomplete for new session path picker

[chphch]

Closes #44.

What this adds: Live subdirectory autocomplete in the new-session "Project path" picker. As soon as the user types a path (e.g. /home/user/proj) the picker fetches matching subdirs from the selected machine via machineBash (ls -1ap) and shows them above the existing "Recent" list; tapping a suggestion appends / so the user can keep drilling down. Debounced 250ms, per-machine cache (10s TTL), no network or disk hit when no machine is selected or the field is empty.

Proof

Screencast of typing a partial path → suggestions appear → tap → drill down: video pending — will attach before requesting human review per CONTRIBUTING.md.

How it works

User types: /home/user/proj
        ↓
parentDir = /home/user/   prefix = proj
        ↓
machineBash: ls -1ap /home/user/ | grep '/$' | grep -v '^\.\.?/$'
        ↓
Filter by prefix → ["projects/", "project-alpha/"]
        ↓
Show as suggestions; tap → sets value to /home/user/projects/

Scope

• 3 files, +174 / -0 — pure addition, no existing behaviour changes when the field is empty or no machine is selected.
• No new dependencies.

[chphch]

Proof — directory autocomplete drills correctly

Captured locally via standalone happy-server (PGlite, port 3055) + Expo web (port 8088), authenticated through getDevWebQueryCredentials() dev URL bypass, driven headlessly with Playwright.

useDirSuggestions calls out to machineBash(machineId, 'ls -1ap "<dir>"', dir) — which requires a connected happy-cli daemon. To capture the UI flow without bringing up a CLI, the hook's fetchDirs was tap-injected at the window.__HAPPY_DIR_MOCK__ boundary so a deterministic mock filesystem stands in for the real ls output. No production code path changed; the mock is removed after capture.

The GIF cycles three states:

1. /home/user/proj typed → Suggestions: /home/user/projects, /home/user/project-alpha, /home/user/project-beta (filter by proj prefix).
2. Tap a suggestion → path appended with trailing /, drilled into the selected directory.
3. /home/user/projects/happy typed → Suggestions: /home/user/projects/happy, /home/user/projects/happy-cli, /home/user/projects/happy-server (filter by happy prefix at the new depth).

Mock filesystem (substitutes for real ls output):

{
  '/home/user/':            ['projects', 'project-alpha', 'project-beta', 'photos', 'docs'],
  '/home/user/projects/':   ['happy', 'happy-cli', 'happy-server', 'web-app', 'mobile-app'],
}

Production behaviour matches: with a real machine connected, useDirSuggestions calls machineBash with the same ls -1ap | grep '/$' | grep -v '^\.\.\?/$' pipeline, debounced 250 ms with a 10 s per-(machineId, parentDir) cache, and feeds the same prefix-filter into the same <Suggestions> UI rendered above the existing Recent list.

Capture details

• Workflow: ~/.claude/skills/happy-pr-proof-capture v0.3.0.
• Mocking: 1-line tap added to useDirSuggestions.fetchDirs for the duration of capture, removed before commit. The hook's debounce/cache/prefix-filter logic runs unchanged — only the ls results are stubbed.
• Same gh release upload pr-proofs + gh pr comment --body-file chain used for feat: add 'max' effort level and propagate effort setting to Claude SDK #1063 / feat(app): star/bookmark conversations with pin-to-top #1078 proofs.