feat(gh-aw): auto-cast and guide resumable work from a single issue #1689

Problem

Starting Squad from a fresh repo requires tribal knowledge:

1. Users must know to create a separate Cast issue before a work-intent issue, then switch issues to run /squad cast, wait for a PR, merge, and finally return to the original issue.
2. Partial activations and missing artifacts produce no recovery guidance — the user is left to guess what happened and what to run next.

Neither of these is discoverable from a first-time-evaluator perspective.

Desired Journey

User posts any normal work command (/squad research, /squad plan, etc.) on one issue. No separate Cast issue. No /squad cast command in normal flow.

1. User posts /squad research (or any work command) on their issue.
2. Squad detects .squad/ is absent.
3. Same run auto-pivots to Cast mode.
4. Squad posts two markers in sequence:
     <!-- squad-pending-intent-v1 -->   ← immutable; records original command
     <!-- squad-cast-opened-v1 -->      ← immutable; links PR when available
5. Cast PR body references: original issue number + original command.
6. User merges the Cast PR.
7. User reruns their original command on the same issue.
8. Squad detects .squad/ present + pending-intent marker → resumes.

First-run safe-output constraint: a same-run comment cannot embed the Cast PR number because the PR mutation is deferred and the number is not yet known. The first-run comment instructs the user to check the repository's Pull Requests tab. On rerun (before merge), Squad can query for the open Cast PR and post the direct link.

Universal Response Invariants

Every Squad comment must satisfy all four:

Additional rules:

• No success without verification. A stage is ✅ only after its artifact is confirmed present. If absent → ⚠️ with exact recovery instruction.
• Additive idempotent retries. Re-running any non-acceptance step is safe and additive. Activation resumes from the last confirmed item.
• Exact N/M counts. Partial runs always report N of M {items} created so far against the plan's declared total, not a system cap.
• Squad owns product failures. Infrastructure failures are distinguished from Squad logic failures. User input is never blamed unless provably the sole cause.
• Marker names are implementation details. Never shown in user-facing copy on success paths.

Scope

In scope

Explicit non-goals

• No install redesign. gh aw add bradygaster/squad/workflows/squad.md@dev is the approved install path. No changes.
• No automatic post-merge execution. Squad does not self-trigger after a Cast PR merges. The user reruns the original command.
• No new relay workflow or cross-workflow triggers.
• No dashboard or status UI.
• No blanket delegation of Cast to a sub-agent.

Acceptance Criteria

Evidence / Release Gate

Before this issue can be marked done:

1. Fresh-repo E2E walkthrough — screencast or step-by-step log showing AC-1 through AC-4 with exact comment text.
2. Structural test suite passes — all tests in AC-5 through AC-8 green.
3. No regression on existing Cast + plan + activate flows in repos that already have .squad/.