ACP: Windows process-tree support (job objects / taskkill, win32 branching, safe-env keys) — dropped from the #2599 split #2805

Context

The PR #2599 split (ACP/Devin provider integration, landed as the 10-PR stack ending with #2795) deliberately shipped POSIX-only process management. Windows support was the most frequently dropped item across the slice PRs — called out in #2689, #2698, #2699, and #2711 (the 8/10 slice, landed as #2783/#2786/#2790/#2792). This issue tracks everything needed to bring the ACP runtime up on Windows.

Current state (verified on dev @ 6e971c5)

• packages/daemon/src/lib/acp/acp-process-tree.ts exists but is POSIX process-group only. The only win32 awareness anywhere is the probe in acpProcessGroupAlive (process.kill(platform === 'win32' ? pid : -pid, 0)); termination is process.kill(-pid, ...) group signaling, which does not exist on Windows.
• acp-transport.ts already spawns with detached: process.platform !== 'win32' and windowsHide: true, so the transport side is Windows-ready in shape, but the kill path is not.
• The daemon-side safe-env allowlist in acp-command.ts retains the Windows keys (USERPROFILE, APPDATA, LOCALAPPDATA, TEMP, TMP, SystemRoot, SystemDrive, PATHEXT, COMSPEC).

Work items

1. acp-process-tree-windows.ts — restore the Windows process-tree owner (job-object and/or taskkill FFI path) dropped from feat(daemon): ACP terminal manager with POSIX process-tree cleanup #2699, plus the win32 branching in acp-process-tree.ts and acp-terminal-manager.ts (spawn/escalation paths).
2. win32 case-insensitive env-key normalization in buildAcpProcessEnv (dropped from the 8/10 slice as "speculative portability" while cleanup was POSIX-only — it stops being speculative once item 1 lands).
3. Windows path-separator parsing tests (shared + daemon) dropped from feat(shared): ACP command spec parsing and identity #2689.
4. Platform-conditional spawn/escalation test assertions in the terminal-manager suite were replaced with POSIX-only ones; restore the platform matrix.
5. Confirm the shared packages/shared/src/acp/ command parsing handles Windows path separators in behavior, not just tests.

Origin

• feat(shared): ACP command spec parsing and identity #2689 "Intentionally dropped": Windows-only safe-env keys, Windows path-separator parsing tests.
• feat(daemon): ACP provider command configuration #2698 "Intentionally dropped": Windows-only process-tree kill hardening.
• feat(daemon): ACP terminal manager with POSIX process-tree cleanup #2699 "Intentionally dropped": acp-process-tree-windows.ts, all win32 branching, platform-conditional test assertions.
• feat(daemon): wire Devin callbacks into the ACP query runtime #2711 "Intentionally dropped": win32 case-insensitive env-key normalization in buildAcpProcessEnv.

Related: #2782 (ACP security hardening track). Parent: #2599.