feat(github): inform sessions of pull request status

[akonwi]

Summary

• add a canonical renderer-neutral pull request status model covering checks, required checks, reviews, approvals, and conservative merge readiness
• extend the daemon GitHub observer with repository-generation fencing and a bounded, consecutively deduplicated rich-status transition stream
• deliver each meaningful transition to the owning Droid through passive, provenance-labeled Inform boundaries without waking idle sessions or duplicating footer/plugin VCS events
• document the behavior and complete CORE-GH-002; leave opt-in automatic reactions to CORE-GH-003

How it works

flowchart LR
    GH["GitHub CLI<br/>pr view + required checks"]
    Cache["Daemon cache<br/>20-second refresh"]
    Observer["VCS observer<br/>canonical PR status"]
    Dedupe{"Changed from<br/>previous state?"}
    Footer["Footer stream<br/>PR number + URL"]
    Inform["Droid.Inform<br/>passive boundary"]
    Droid["Droid context<br/>next safe boundary"]

    GH --> Cache --> Observer
    Observer --> Footer
    Observer --> Dedupe
    Dedupe -- No --> Ignore["Ignore duplicate poll"]
    Dedupe -- Yes --> Inform --> Droid

Loading

Distinct transitions are preserved, including a return to an earlier state:

pending ──▶ failed ──▶ pending ──▶ passed
   │           │          │          │
 Inform      Inform     Inform     Inform

pending ──▶ pending
   │           ╰─ unchanged: ignored
 Inform

File ownership:

internal/
├── pullrequest/status.go       canonical renderer-neutral model
├── githubpr/
│   ├── pull_request.go         bounded GitHub queries + normalization
│   └── cache.go                20-second cache, repository generations
├── vcs/
│   ├── status.go               local Git/repository identity
│   └── observer.go             footer + rich transition streams
├── server/vcs_host.go          projects observer into session ports
└── session/
    ├── github_status.go        validates and calls Droid.Inform
    └── manager.go              owns informer lifecycle

Delivery remains passive:

Status changes while idle
  └─ boundary remains pending
     └─ consumed on next user turn

Status changes during active work
  └─ boundary waits for a safe model boundary
     └─ Droid can incorporate it into the current turn

No automatic turn is started
  └─ CORE-GH-003 will own ask/auto-react policy

Validation

• gofmt -l .
• git diff --check
• go build ./...
• go vet ./...
• go test ./...
• go test -race ./...
• go test -race ./internal/githubpr ./internal/vcs ./internal/session ./internal/server

All validation used isolated temporary KIT_HOME directories. No development daemon or production Kit state was used.