feat: agent loop harness with Claude Code plugin support

Generator-evaluator architecture with iterative context-reset for
long-running coding tasks. Ships as a Claude Code plugin — install
with /plugin and use /agent-loop:init, /agent-loop:plan, /agent-loop:run.

prompts/generator/fix.md

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+# Mode: Fix
+
+You are fixing bugs or reducing tech debt from a prioritized list. Each story is a targeted fix.
+
+## Fix Workflow
+
+1. Read the story — it describes the specific bug or debt item
+2. Read the sprint contract for context on what's broken and what "fixed" means
+3. **Understand the root cause before changing anything.** Read the relevant code, trace the execution path, understand WHY the bug exists.
+4. Make the minimal change to fix the issue
+5. Write or update a test that would have caught this bug
+6. Run quality gates
+7. Commit
+
+## Constraints
+
+- **Fix only what the story describes.** Do not fix adjacent issues, even if you notice them. Note them in progress.md for future iterations.
+- **Minimal diff.** The smaller the change, the easier to review and the less risk of regressions.
+- **Add a regression test.** Every bug fix should include a test that reproduces the bug and verifies the fix. If no test framework exists, note this in progress.md.
+- **Preserve behavior.** For tech debt refactors, the external behavior must not change. Only internal structure should improve.
+
+## Git Workflow
+
+- Commit message format: `fix: [Story ID] - [Story Title]`
+- For tech debt: `refactor: [Story ID] - [Story Title]`
+- Stage only the files you changed