fix(pipeline): enforce staged-file scope in implement step — prevent git add -A

[nextlevelshit]

Problem

The implement step's craftsman persona ran git add -A (or equivalent), staging 3280 files — the entire worktree — instead of only the files it modified for the issue. The prompt explicitly says "Do NOT use git add -A or git add . — stage specific files only" but the model ignored this instruction.

Observed with opencode adapter on impl-issue run for #1622. The implement prompt has this constraint but no enforcement.

Impact

• PRs contain entire repo diff instead of targeted changes
• test_diff and test_count_baseline contracts can't detect test deletions (baseline is polluted by unrelated files)
• Review quality degrades — 3280-file diff is unreviewable
• Wastes git storage and CI time

Proposal: staged-file scope enforcement

Add a post-implement contract that validates the commit scope:

1. New contract type: commit_scope — after implement step commits, check that the diff contains only files related to the issue. Implementation options:

   • Option A (cheap): Check file count threshold (e.g., max_files: 50). If exceeded → rework. Catches the git add -A case.
   • Option B (smart): Compare committed files against the impl-plan.json files_to_modify list. Any file not in the plan → warning or rework.
   • Option C (hybrid): Threshold check first (Option A), then plan-aware check if threshold passes (Option B).

2. Prompt hardening: Add a stronger constraint in implement.md and fix-implement.md:

   CRITICAL: After committing, run `git diff --name-only HEAD~1`. If more than 10 files
   appear, you staged too much. Run `git reset HEAD`, re-stage only your changes,
   and re-commit.
   

3. Rework loop integration: commit_scope contract with on_failure: rework feeds into the existing fix-implement step.

Recommended: Option A (threshold) + prompt hardening

Option A is cheap, catches the common case, and doesn't require plan-schema changes. Prompt hardening adds a self-check. Option B/C can be added later if threshold false-positives occur.

# impl-issue.yaml implement step handover:
- type: commit_scope
  max_files: 50
  on_failure: rework

Files to touch

• internal/contract/commit_scope.go — new contract type
• internal/contract/registry.go — register commit_scope
• internal/defaults/embedfs/pipelines/impl-issue.yaml — add to implement handover
• .agents/pipelines/impl-issue.yaml — same
• internal/defaults/embedfs/prompts/implement/implement.md — add self-check instruction