Add thin Windows PowerShell launcher

[lucianlamp]

Summary

Windows PowerShell users currently depend on session-local aliases/functions or ad hoc wrappers. This adds a thin PowerShell launcher that delegates to the existing Bash scripts, preserving agmsg's single implementation and DB access boundary while making Windows invocation reliable.

This PR also folds in the Codex sandbox README note so the Windows follow-up and README-only update land together.

What changed

• Add scripts/dispatch.sh as the Bash-side command dispatcher for inbox, send, history, team, config, mode, join, reset, actas, and drop.
• Keep scripts/windows/agmsg.ps1 as a thin Windows platform shim: Git Bash detection, UTF-8 setup, sqlite3 preflight, argv handoff, then dispatch.sh.
• Remove the previous Windows runner/sqlite3-shim approach and clean up legacy shim files only when they match the agmsg marker.
• Keep PowerShell integration under the skill tree: install.sh no longer writes a top-level ~/.agents/<cmd>.ps1; users can enable a profile function via scripts/windows/install-agmsg.ps1.
• Add optional PowerShell profile installer docs and Windows usage docs.
• Add Codex sandbox writable-roots guidance to README.
• Fix install.sh --cmd <name> --update so it updates the named skill instead of the first .agmsg skill directory found.
• Deflake the watcher self-clean install smoke by waiting for the pidfile owner to change instead of relying on a fixed short sleep.

Test plan

Local checks run on Windows:

• git diff --check
• bash -n install.sh uninstall.sh scripts/delivery.sh scripts/dispatch.sh scripts/send.sh scripts/inbox.sh scripts/history.sh scripts/team.sh scripts/join.sh scripts/reset.sh scripts/whoami.sh scripts/identities.sh
• PowerShell parser check for scripts/windows/agmsg.ps1 and scripts/windows/install-agmsg.ps1
• npm test
• powershell -NoProfile -ExecutionPolicy Bypass -File tests/smoke_windows_powershell.ps1 -RepoRoot (Resolve-Path .).Path
• Manual Git Bash dispatcher smoke covering explicit/env/whoami identity, multiple identity refusal, Japanese + quotes + emoji send/history, and mode off / mode turn
• Manual install/update cleanup smoke, including no top-level ~/.agents/<cmd>.ps1 creation and cleanup of the earlier generated shortcut
• Manual uninstall cleanup smoke
• Real installed PowerShell E2E using a profile-managed agmsg function that points at ~/.agents/skills/agmsg/scripts/windows/agmsg.ps1: join -> send -> history -> inbox -> env identity -> whoami identity -> mode off/turn -> team -> reset

CI is green on the latest head:

• check: pass
• bats (ubuntu-latest): pass
• bats (macos-latest): pass

[lucianlamp]

CI is green on the latest head (6a59dcd):

• check: pass
• bats (ubuntu-latest): pass
• bats (macos-latest): pass

I also ran a real native Windows PowerShell E2E after installing this branch into ~/.agents/skills/agmsg with install.sh --cmd agmsg --update and dot-sourcing ~/.agents/agmsg.ps1.

Covered flow:

join -> send -> history -> inbox -> env identity -> whoami identity -> mode off/turn -> team -> reset

The message body included Japanese text, quotes, and emoji (実機E2E 確認しました "quoted" emoji 🚀) and was preserved through history / inbox. The E2E used a temporary AGMSG_STORAGE_PATH and cleaned up the temporary team with reset.

[lucianlamp]

One design note compared with #103:

This follow-up intentionally simplifies the native-Windows approach from the previous PR. Instead of keeping a Windows runner script plus a sqlite3 compatibility shim, the current version keeps Windows-specific code to a thin PowerShell platform shim:

• detect Git Bash
• set UTF-8-oriented process settings
• preflight sqlite3 from Git Bash
• preserve argv across the PowerShell -> Git Bash boundary
• delegate into the Bash-side scripts/dispatch.sh

All command semantics and DB access stay on the Bash side (dispatch.sh -> existing scripts/*.sh). The Git Bash installer also no longer writes a top-level ~/.agents/<cmd>.ps1; PowerShell integration is enabled through the profile installer under the skill tree (scripts/windows/install-agmsg.ps1).

So compared with #103, this PR removes the runner/shim layer and makes the Windows path smaller: PowerShell is only the launcher, Bash remains the implementation.

[fujibee]

Reviewed — this looks good. The design is clean: keeping all command semantics in dispatch.sh so the PowerShell side is a thin platform shim (Git Bash detection, UTF-8, sqlite3 preflight, then delegate) preserves agmsg's single-implementation boundary, and the base64 + file argv handoff is the right way to get spaces / quotes / Japanese / emoji across the PowerShell→Bash boundary without multi-layer shell re-parsing. The marker-guarded cleanup of the legacy shim/shortcut (only removing files that match the agmsg marker) and the --cmd <name> --update fix (targeting the named skill instead of the first .agmsg dir) are both correct improvements. The #124 deflake bundled in is a nice extra.

CI is green (check + bats ubuntu/macos), and the manual Windows E2E in the test plan is thorough. Approving.

Two non-blocking notes for follow-up:

1. Second command surface. dispatch.sh now routes commands in parallel with the per-agent skill templates (cmd.*.md). Adding a future command means updating both, so they can drift. Not a blocker — a reasonable cost for giving Windows/shell users a real CLI — but worth keeping in mind, or eventually having the templates reference dispatch where practical.

2. PowerShell side isn't covered by CI. scripts/windows/agmsg.ps1 and tests/smoke_windows_powershell.ps1 only run via the manual E2E here — there's no windows-latest runner yet (CI: add Windows (windows-latest) coverage for the native Windows helpers #125). The Bash/dispatch side is CI-covered; landing CI: add Windows (windows-latest) coverage for the native Windows helpers #125 would lock the PowerShell path in too so it can't regress silently.

Thanks for the thorough test plan.