Windows support: unblock the POSIX-only assumptions

[raysonmeng]

Goal

AgentBridge currently runs on macOS and Linux only; the README states Windows is not officially supported. This tracks making Windows a first-class platform. The codebase is already partly platform-aware (port cleanup has a win32 path, issue #76), so this is about closing the remaining POSIX-only gaps, not a rewrite.

Known blockers (from a source scan)

1. Unix-domain-socket transport — src/codex-transport.ts falls back to a unix:// socket (<tmp>/agentbridge-<uid>/codex-<port>.sock) for Codex builds that dropped ws:// listen support. Windows has no AF_UNIX in Node the way POSIX does (path/net semantics differ). Needs a named-pipe (\\.\pipe\...) path or a forced-ws:// strategy on win32.

2. Process-group signals — src/process-lifecycle.ts uses -pid process-group kills and POSIX signals (SIGTERM/SIGKILL). Already guarded with process.platform !== "win32" in places, but the win32 branch needs a real termination path (taskkill /T) verified end to end, not just a no-op fallback.

3. State directory — src/state-dir.ts handles darwin (~/Library/Application Support) and Linux ($XDG_STATE_HOME), but has no Windows branch. Should use %LOCALAPPDATA% (fallback %APPDATA%).

4. CI has no Windows gate for the full suite — .github/workflows/ci.yml matrix is [ubuntu-latest, macos-latest]; only a narrow windows-port-cleanup job runs on windows-latest. Without a full Windows CI leg, regressions on win32 are invisible.

5. Runtime / launcher on Windows — the bin shebang (#!/usr/bin/env bun) and the abg claude / abg codex launchers assume a POSIX shell + Bun on PATH. Needs verification that Bun + the plugin host + Codex CLI actually launch and attach on Windows (PowerShell / Windows Terminal), including any path-separator assumptions.

Suggested order

1. State dir (%LOCALAPPDATA%) — smallest, unblocks daemon state.
2. Transport: named pipe or force-ws:// on win32.
3. Process termination via taskkill /T /F, verified.
4. Add a full-suite windows-latest CI leg (expect it red first, then fix to green).
5. End-to-end smoke: abg init → abg claude → abg codex on Windows Terminal.

WSL2 likely works today (it's Linux) and can be documented as an interim path while native win32 lands.

Definition of done

• Full test suite green on windows-latest in CI.
• abg init / claude / codex complete a real pair on native Windows.
• README limitation line removed; Windows listed as supported.