Implement a plan

Executes a plan already authored by plan-doc at specs/NNN-<slug>/plan.md, then hands the finished work to finalize to close out. Follows the plan exactly; flags ambiguity instead of guessing.

When to use this

• A specs/NNN-<slug>/plan.md exists with status: authored.
• Skip if no plan exists yet — run plan-doc first.

Procedure

1. Read .specify/memory/constitution.md, then the plan in full. Identify every deliverable. If the user named a plan, read it directly; otherwise take the one with status: authored from specs/README.md and confirm which to build.

2. Work the plan's Steps via the constitution's Item-execution loop — in order, Do then Verify, one at a time. On an ambiguous or conflicting step, park a gate (specs/NNN-<slug>/gates/NNN-<what>.md) rather than guessing; you are long-running, so parking is fine.

3. Write tests alongside each unit, matching the style already in the codebase (look at one existing test file first). Run them as part of that step's Verify.

4. Close out via /finalize — done-checks, changelog, version, plan-index flip to implemented, commit on confirm. Don't duplicate any of that here. (A setup with its own ledger contract runs its overlay — e.g. /land-feature — instead.)

What not to do

• Don't invent scope beyond the plan's Steps — new ideas go back to plan-doc.
• Don't make architectural decisions mid-build — park a gate (step 2).
• Don't skip a step's Verify because it "obviously works".
• Don't assert a function's return shape from memory — read it first. When the test is about absence, assert shape-independently (serialize the payload and search it).