feat(agent): auto-match PRs to open GitHub Milestones (suggest-mode)

[JSONbored]

Parent epic: #3180

Goal

Detect when a PR (even with no closing-keyword issue link) is likely part of an open GitHub Milestone, and post a bot-comment suggestion — never auto-apply in this phase.

Requirements

• New tri-state setting autoProjectMilestoneMatch: "off" | "suggest" | "auto" (default "off"), mirroring the reviewCheckMode tri-state template (migrations/0107_repository_review_check_mode.sql, src/signals/focus-manifest.ts:1042-1057). Only "off"/"suggest" are meaningful until feat(agent): auto-match PRs to open GitHub Projects v2 (suggest-mode) #3184 lands.
• List open milestones via GET /repos/{owner}/{repo}/milestones?state=open.
• Match PR title/body tokens against milestone titles by reusing the existing collision-detection primitives in src/signals/engine.ts: tokenize (:5301), termOverlap (:5273) — use the same or a stricter threshold than the existing duplicate-PR gate (score >= 0.58 && shared >= 2, :865), since a wrong milestone attach corrupts tracked progress rather than just adding an advisory note.
• Also detect a repo-configured branch-name short-code convention (e.g. <code>-<number>-<slug>) as a high-precision secondary signal — this mirrors the convention Linear's own GitHub integration already uses, so it doubles as prep for feat(agent): Linear adapter for project/milestone matching #3186 (Linear adapter).
• In "suggest" mode: post a single bot comment naming the matched milestone and confidence; never call the milestone-set API.
• Hook alongside the existing linked-issue evidence-gathering step (buildPullRequestAdvisory, src/queue/processors.ts:5239), not inside the hard-rule blocker path (src/review/linked-issue-hard-rules.ts:71,140) — a missed/wrong match must never fail the gate.
• Define the ProjectTrackerAdapter interface (new file, e.g. src/integrations/project-tracker-adapter.ts) even though only the GitHub-milestone half is implemented here, so feat(agent): auto-match PRs to open GitHub Projects v2 (suggest-mode) #3184 (Projects v2) and feat(agent): Linear adapter for project/milestone matching #3186 (Linear) slot in without reshaping this code:

  interface ProjectTrackerAdapter {
    listOpenProjects(ctx): Promise<{ id: string; title: string }[]>;
    listOpenMilestones(ctx): Promise<{ id: string; title: string }[]>;
    attachToProject(ctx, prRef, projectId): Promise<AttachResult>;
    attachToMilestone(ctx, prRef, milestoneId): Promise<AttachResult>;
  }

Deliverables

• ProjectTrackerAdapter interface + GitHubMilestonesAdapter (milestone half only).
• Matching logic reusing engine.ts tokenize/termOverlap.
• Suggest-mode bot comment.
• Config: tri-state setting + .gittensory.yml parity + OpenAPI.
• Tests: match / no-match / ambiguous-multi-match cases, both branches of the tri-state default fallback.

Expected outcome

Repos that opt in get a visible, non-blocking suggestion comment identifying which open milestone a PR probably belongs to, with zero risk of misfiled tracking data since nothing is auto-applied yet.

[linear-code]

JSO-2613