Add /story skill — walk ordered sub-issues of a story ticket within one session

[sebastientaggart]

Problem to Fix

Even with smoother per-ticket primitives, working through a multi-ticket story still requires the operator to manually invoke /start <next> after each merge.

Why it Matters

Completes the "automate a little more" arc. Builds on the earlier children so the driver itself stays small and conservative.

General Approach

/story <issue#> reads the parent issue's sub-issues in their GitHub-defined order. For each open sub-issue: invoke /start <number>, then the (now-tiered) /submit-for-review. Continue conditions: clean validation, unambiguous AC, no auth/payments/secrets/prod/destructive touch, follow-ups resolved per the follow-up-spawning child. Stop at phase boundaries (TBD: how phases are defined within sub-issues — resolve during implementation). Per-ticket attempt cap of 2; no-progress guard on empty/identical diffs. State is derived from sub-issue status; phase summaries posted as informational comments only.

Complexity

Verification / QA effort: extensive
Concurrency-free but behaviorally rich; needs explicit verification of each escalation trigger and continue condition. Depends on the prior three children.

Acceptance Criteria

• /story <issue#> parses sub-issues in order and echoes the plan before starting.
• With clean validation, the driver advances between tickets within a phase without prompting.
• The driver always stops at phase boundaries.
• Each escalation trigger (validation failure, auth/payments/secrets/prod/destructive, scope expansion, unclassifiable follow-up) produces stop-and-ask when induced.
• No claude -p or external process invoked at any point.
• Story state is reconstructible from sub-issue status alone — deleting/editing comments does not break resumption.

Parent story

Part of #167. Depends on children A, B, and C.

[sebastientaggart]

Resolution

The /story driver is now available. Invoking it on a parent ticket with sub-issues walks the open ones in order, threading the per-ticket flow into a single session and removing the two routine prompts that fired between merges. The deliberate human-judgment moments from the per-ticket flow — actionable warnings and blocking findings during review — still stop and ask. State is derived from sub-issue status, so an interrupted story resumes cleanly with a fresh /story invocation.

See #178 for full technical details.