Integrate planning phases and require implementation pull requests

[rochecompaan]

Part of #180.

Blocked by: #188

Goal

Integrate the phase coordinator into run-once and require a validated implementation pull request before finishing an issue.

Scope

• Add issue selection priority and cross-process locking.
• Integrate all four planning gate combinations.
• Add review-pending and explicit stop result mappings and exit codes.
• Force allowDirectLand: false for the new workflow.
• Validate the implementation pull request’s target, head, marker, and commit ancestry.
• Run finish-stage cleanup only after implementation pull request validation succeeds.

Out of scope

• Deprecated command migration.
• Installed skill-pack and user documentation updates.
• Final cross-provider acceptance scenarios.

Acceptance criteria

• run-once stops at each required human-review gate.
• The implementation result cannot bypass pull request validation.
• Redirected JSON, terminal summaries, and exit codes cover every new result.
• Legacy workflow behavior remains unchanged until compatibility work lands.

[rochecompaan]

This was generated by AI during triage.

Triage Notes

This issue is clear and suitable for automation, but its implementation depends on the still-open same-repository issue #188. Re-evaluate it for agent-ready after that blocker closes.

Blocked by: #188