Agent-backed VCS writes #205

Parent

Part of #154 (remote agent sessions). Next Steps item 3 of docs/designs/remote-workrooms.md.

What to build

Route VCS write operations (commit and other mutating VCS actions) through wr-agent instead of native local providers, giving write parity with the read path landed in #204. Preserve existing typed failure surfaces and Retry/Abort behavior. Handle remote clones whose workroom root and project root are the same repository without inventing a second coordination root.

Acceptance criteria

• Writes route through the agent connection for registered repositories; unregistered repositories still refuse writes as today.
• Typed failures and Retry/Abort behavior match pre-agent behavior exactly.
• A remote clone whose project root equals its workroom root commits correctly, with no duplicate or conflicting coordination root.
• JJ snapshot ordering and the filesystem write barrier from feat(macapp): route local VCS reads through the agent #204's read path are respected by writes too.
• Integration coverage drives the bundled agent through the real Swift client against throwaway repositories for the write path, mirroring feat(macapp): route local VCS reads through the agent #204's approach for reads.

Blocked by

None — stacks directly on feat/agent-vcs-reads (#204), which already exists. Can start now.