feat(gastown): extend GitHubPRStatusSchema with mergeable_state and wire conflict detection

[jrf0110]

Summary

• Extends GitHubPRStatusSchema in platform-pr.util.ts to capture mergeable and mergeable_state fields from the GitHub API response
• Updates checkPRStatus in town-scm.ts to return a PRStatusResult object ({ status, mergeable_state }) instead of a plain string, so callers can access the conflict state
• Wires conflict detection into the poll_pr action: when mergeable_state === 'dirty', emits pr_conflict_detected exactly once per conflict episode (tracked via has_conflicts flag on MR bead metadata); clears the flag when state transitions to clean/unknown
• Adds applyEvent('pr_conflict_detected') handler in reconciler.ts that creates a gt:pr-conflict issue bead (with rebase/resolve instructions) when auto_resolve_merge_conflicts is enabled, or an escalation bead when disabled; idempotent via hasExistingPrConflictBead check
• Handles gt:pr-conflict bead lifecycle in review-queue.ts agentDone path: closes the bead directly (skip review queue), unblocking the parent MR bead for continued polling
• Adds pr_conflict_detected to TownEventType enum

Verification

• TypeScript types align: PRStatusResult imported in actions.ts, all checkPRStatus call sites updated
• No as casts or ! assertions; auto_resolve_merge_conflicts read with z.safeParse for forward-compatibility with the parallel schema bead
• TownEventType enum updated so insertEvent('pr_conflict_detected', ...) validates correctly

Visual Changes

N/A

Reviewer Notes

• The auto_resolve_merge_conflicts config field is being added by a parallel bead in this convoy (cce77f31). Both actions.ts and reconciler.ts use z.safeParse with optional() and default to true if the field is absent — this is the safe forward-compatible approach.
• The checkPRStatus return type change is a breaking change to ApplyActionContext. All callers have been updated (poll_pr destructures { status, mergeable_state }, the fresh-status re-check uses .status).
• gt:pr-conflict beads block the parent MR bead via bead_dependencies (same pattern as gt:pr-feedback), so the MR bead won't be processed while conflicts are outstanding.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (6 files)

• services/gastown/src/db/tables/town-events.table.ts
• services/gastown/src/dos/town/actions.ts
• services/gastown/src/dos/town/reconciler.ts
• services/gastown/src/dos/town/review-queue.ts
• services/gastown/src/dos/town/town-scm.ts
• services/gastown/src/util/platform-pr.util.ts

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_{Reviewed by gpt-5.4-20260305 · 581,892 tokens}