feat: add P1 work-item delivery orchestration

[Wibias]

Summary

• add tracker-agnostic work-item delivery state planning, with Linear as the first normalized tracker shape
• derive tracker milestones from verified GitHub delivery evidence instead of hardcoded tracker status names or IDs
• fail closed on missing or ambiguous review/status mappings
• bind tracker reconciliation plans to the observed current status ID so stale writes must re-plan
• route external work-item requests through a dedicated controller without allowing tracker content to grant merge authority
• expose external work-item delivery in SKILL.md discovery metadata so the host can select the skill before the internal router runs

Hardening found during self-review

• work on ENG-42 and open a PR now binds the explicitly requested push_code and create_pr actions
• implementation-only requests such as implement ENG-42 do not silently gain publication authority
• ship ENG-42 carries publication plus explicit merge intent into delegated phases without weakening the normal merge gate
• review-like tracker names are accepted for the review milestone only when the tracker type is started, so a completed status such as Review Complete cannot be selected accidentally

Stack

• depends on feat: add P0 open-work and publication safety #283 (feat/p0-open-work-publication-safety)
• next P1 slices stack on this branch

Validation

• focused unit contracts cover work-item keys, milestone derivation, ambiguous status maps, tracker-type safety, expected-status transitions, covering-PR reuse, publication-action binding, routing, mutation modes, and controller phases
• repository-wide local suite is intentionally left to the user's local run; GitHub Actions is not used as the current gate

Provenance

The lifecycle concept was informed by OutThisLife/brooklyn-skills ticket-ship and redesigned for GitHub Delivery's existing evidence, publication, and merge gates.

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