feat: three more agents, tool allowlists, and a README that records the tiers

Committed as-authored to establish a recoverable baseline before the
client-neutral architecture port rewrites these paths. No content is changed
here; this is the working tree as it stood.

- `plan-reviewer`, `pr-creator` and `release-notes` join the set.
- `refactor-scan` and `security-scanner` gain explicit `tools:` allowlists, so a
  review agent can no longer edit or write.
- The README's agent table records each agent's model tier and read-only status,
  and documents how to invoke the three new ones.

One gap is left as-is rather than fixed mid-baseline: `pr-creator` declares no
`tools:`, so it inherits the full tool pool while every sibling review agent is
constrained. It is also the only agent here that legitimately needs to write.

.claude/README.md

@@ -28,11 +28,14 @@ A comprehensive guidance system for Claude Code with strict TDD enforcement, mul
 ├── CLAUDE.md                    # Core principles (~150 lines)
 ├── settings.json                # Model config (opusplan), Claude hooks
 ├── agents/
-│   ├── tdd-guardian.md          # TDD enforcement
-│   ├── code-reviewer.md         # PR review (uses Opus)
-│   ├── security-scanner.md      # Security checks
-│   ├── refactor-scan.md         # Refactoring assessment (TDD step 3)
-│   └── dependency-audit.md      # Vulnerability/outdated package checks
+│   ├── tdd-guardian.md          # TDD enforcement (sonnet)
+│   ├── code-reviewer.md         # PR review (opus)
+│   ├── security-scanner.md      # Security checks (sonnet, read-only)
+│   ├── refactor-scan.md         # Refactoring assessment (sonnet, read-only)
+│   ├── dependency-audit.md      # Vulnerability/outdated package checks (sonnet)
+│   ├── pr-creator.md            # Automated PR creation (haiku)
+│   ├── release-notes.md         # CHANGELOG generation (haiku, read-only)
+│   └── plan-reviewer.md         # Plan validation (sonnet, read-only)
 ├── skills/
 │   ├── languages/
 │   │   ├── python/SKILL.md      # pytest, Ruff, FastAPI
@@ -173,6 +176,21 @@ echo '~/.claude/hooks/pre-commit-tdd.sh' > .husky/pre-commit
 @dependency-audit Check for vulnerable or outdated packages
 ```
 
+### Create Pull Request
+```
+@pr-creator Create a PR for this branch
+```
+
+### Generate Release Notes
+```
+@release-notes Generate changelog from v1.0.0 to HEAD
+```
+
+### Review Implementation Plan
+```
+@plan-reviewer Review docs/plans/active/3.5_business-capabilities.md
+```
+
 ## Customization
 
 ### Project-Specific CLAUDE.md