Add durable planning state and pull request workspaces

[rochecompaan]

Part of #180.

Blocked by: #184

Goal

Implement the durable local infrastructure used by planning pull request phases.

Scope

• Add strict versioned state with atomic replacement and immutable run identity.
• Add the ownership-ID issue lock and stale-lock diagnostics.
• Implement the Git-backed planning workspace adapter.
• Fetch and pin the remote base before creating each phase workspace.
• Detect planning artifacts already present on the remote base.
• Make resume and owned-workspace cleanup idempotent.

Out of scope

• Running planning agents.
• Creating or reconciling pull requests.
• Pipeline selection and implementation execution.

Acceptance criteria

• State parsing rejects unknown or contradictory documents.
• State writes and lock acquisition are atomic.
• Workspace creation starts from the fetched remote base commit.
• Resume reuses only saved, matching workspace identity.
• Cleanup removes only Patchmill-owned phase workspaces.

[rochecompaan]

This was generated by AI during triage.

Triage Notes

This issue is clearly scoped and suitable for automation once its concrete same-repository dependency is resolved.

Blocked by: #184