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/planner/plan.md

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+# Planner Context
+
+This file is loaded by the `/loop-plan` skill to provide additional context for PRD generation.
+
+## Story Decomposition Guidelines
+
+When breaking a feature into stories, think about:
+
+### Independence
+Each story should be independently deployable. After completing story N, the codebase should be in a valid, working state — even if the feature isn't fully built yet.
+
+### Context Window Fit
+A story must fit in a single AI context window (~100K tokens). This means:
+- Reading relevant existing code
+- Understanding the task
+- Implementing the change
+- Writing tests
+- Running quality checks
+- Committing
+
+Budget roughly:
+- 30% of context for reading/understanding
+- 40% for implementation
+- 20% for testing and quality
+- 10% for bookkeeping (prd.json, progress.md)
+
+### Failure Isolation
+If a story fails (evaluator rejects it), the next iteration should be able to retry it cleanly. Stories with too many moving parts are hard to retry because partial state is messy.
+
+### Evaluability
+Every story must have criteria the evaluator can independently verify. "The code is clean" is not evaluable. "The function returns 404 when the user doesn't exist" is evaluable.
+
+## PRD Anti-Patterns
+
+Avoid these common mistakes:
+
+- **Stories too large:** "Build the API" — split into individual endpoints
+- **Stories too small:** "Create the file" — combine with meaningful work in that file
+- **Vague criteria:** "Works correctly" — what does correctly mean? Be specific.
+- **Missing dependencies:** Story 5 needs Story 3's database table but doesn't say so
+- **Testing as afterthought:** Tests should be part of each story, not a separate "add tests" story at the end
+- **UI without backend:** A UI story that calls an API that doesn't exist yet