docs: add MACP brief template and Phase 2A test brief

docs/MACP-BRIEF-TEMPLATE.md

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+# MACP Task Brief Template
+
+**Use this template for all MACP task briefs.** Workers that receive briefs not following this structure should flag it as an issue.
+
+---
+
+```markdown
+# <Title>
+
+**Branch:** `feat/<branch-name>`
+**Repo worktree:** `~/src/<repo>-worktrees/<task-slug>`
+
+---
+
+## Objective
+
+<1-2 sentences: what is being built and why>
+
+---
+
+## Task 1: <Component Name>
+
+<Description of what to build>
+
+### Requirements:
+- <Specific, testable requirements>
+
+### Key Functions/APIs:
+<Code signatures or interface definitions>
+
+### Constraints:
+- <Language, dependencies, patterns to follow>
+
+---
+
+## Task 2: <Component Name>
+<Same structure as Task 1>
+
+---
+
+## Tests (MANDATORY)
+
+**Every brief MUST include a Tests section. Workers MUST write tests before or alongside implementation. Tests MUST pass before committing.**
+
+### Test file: `tests/test_<module>.py`
+
+### Test cases:
+1. `test_<name>` — <what it verifies>
+2. `test_<name>` — <what it verifies>
+...
+
+### Test runner:
+```bash
+python3 -m unittest discover -s tests -p 'test_*.py' -v
+```
+
+---
+
+## Verification
+
+1. All tests pass: `<test command>`
+2. Python syntax: `python3 -c "import <module>"`
+3. <Any additional verification steps>
+
+## Ground Rules
+- Python 3.10+ stdlib only (no pip dependencies)
+- Commit message: `feat: <what changed>` (conventional commits)
+- Push to `feat/<branch>` branch when done
+```
+
+---
+
+## Brief Sizing Rules
+
+| Brief Type | Max Items | Rationale |
+|------------|-----------|-----------|
+| **Build** (new code) | 2-3 | High cognitive load per item |
+| **Fix** (surgical changes) | 5-7 | Low cognitive load, exact file/line/fix |
+| **Review** | 1 | Naturally focused |
+| **Test** (add tests) | 3-4 | Medium load, but well-scoped |
+
+The key metric is **cognitive load per item**, not item count.
+- Build = construction (high load)
+- Fix = scalpel (low load)
+- Review = naturally focused
+- Test = moderate (reading existing code + writing test logic)