feat(ui): add /cwd path tab-completion with async directory picker

[douglarek]

The /cwd slash command now exposes `argCompleter: "path"`, a new completer type that async-lists the filesystem via `listDirectory()`. Tab drills into directories (trailing '/'), Enter commits. Matches are capped at SEARCH_RESULT_CAP (200) to match @-mention search.

• types.ts: add "path" to argCompleter union
• commands.ts: wire /cwd to argCompleter: "path"
• useCompletionPickers.ts: new usePathCandidates hook; update pickSlashArg for dir drill-down; expose slashArgPathCandidates
• App.tsx: Tab handler checks candidate.isDir for drill vs. commit
• SlashArgPicker.tsx: optional pathCandidates prop, renders trailing / on directory rows

What

2026-05-15.17.15.17.mov

Why

How to verify

Checklist

• npm run verify passes locally (lint + typecheck + tests + comment-policy gate)
• No Co-Authored-By: Claude trailer in commits
• Comments follow CONTRIBUTING.md (no module-essay headers, no incident history)
• No edits to CHANGELOG.md — release notes are maintainer-written at release time

[esengine]

Nice extension of the argCompleter abstraction — drill-on-/ vs commit-on-space is the right shape, and capping at SEARCH_RESULT_CAP keeps it consistent with @-mention.

Two things before merge:

1. Windows absolute paths are missed. Detection at usePathCandidates uses partial.startsWith("/"), which is POSIX-only. C:\Users\bob or C:/Users/bob skips the absolute branch, gets resolved against rootDir, then immediately falls through the ..-escape path and tries to list from / — wrong twice (root / isn't a real path on Windows, and the canonical separator is \). Use node:path.isAbsolute for detection and path.parse(p).root for the listing root. Windows is the project's primary platform; worth handling properly.

2. /cwd lists files too. The completer offers everything from listDirectory. A user tabbing a .ts row gets it committed with trailing space; Enter then sends /cwd path/to/file.ts which /cwd rejects — the picker shouldn't be offering invalid options in the first place. Easiest fix: filter to isDir only inside the "path" branch (single consumer today, safe). If we later need a file-completer for some other command, introduce argCompleterMode: "dirs" | "files" | "any" then.

A couple of unit tests around drill-down and the escape-to-absolute branch would be nice but not blocking — happy to merge once the two above are in.

[esengine]

Both addressed cleanly — isAbsolute(partial) + parse(partial).root is exactly the shape I wanted, and the backslash→forward-slash normalization keeps parseAtQuery happy on Windows. Dir-only filter on the listDirectory result is the right place to do it.

Heads-up: the matrix CI change landed in #926 about an hour ago, so on your next contribution the build job will run on windows-latest too — the path-handling code in this PR would have been verified end-to-end there. This one's already CI-green on Linux; merging.