Harvest: extension system + before/after phase hooks

[jbaruch]

Status: Next up (Tier 1)

Upstream spec-kit added a first-class plugin model with extensions/<name>/extension.yml manifests, command registration into all agent dirs, a community catalog (catalog.community.json), and a hook system that fires before_<phase> / after_<phase> events from every core command. Hooks can run shell scripts OR dispatch into agent skills.

What we'd build

Hook dispatcher in each iikit-* skill, extension manifest schema, registration logic. Phase template injection is mechanical.

Why it matters

IIKit's value is the BDD-verification chain. Hooks would let third parties plug into IIKit phases without forking:

• coding-policy-review hook on after_implement
• linear-sync hook on after_tasks
• slack-notify hook on after_analyze

Aligns perfectly with the Tessl tile distribution model — extensions ship as tiles.

Evidence

• Modular extension system: f14a47ea (#1551)
• Git extension + 18 hooks: 1a9e4d1d (#1941)
• Wire hooks into specify/plan: 333a7653 (#1886)
• after_tasks/after_implement: d0a112c6 (#1702)
• Claude skill-chaining for hooks: 8fc2bd34 (#2227)
• Docs: extensions/EXTENSION-API-REFERENCE.md

Cost

Medium-Large — hook dispatcher in each iikit skill, manifest schema, registration logic.

Sequencing

Tier 1, next up alongside #(workflow engine). Not for grabs — needs design work.