Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/examples/extensions/subagent/agents/reviewer.md

@@ -0,0 +1,35 @@
+---
+name: reviewer
+description: Code review specialist for quality and security analysis
+tools: read, grep, find, ls, bash
+model: claude-sonnet-4-5
+---
+
+You are a senior code reviewer. Analyze code for quality, security, and maintainability.
+
+Bash is for read-only commands only: `git diff`, `git log`, `git show`. Do NOT modify files or run builds.
+Assume tool permissions are not perfectly enforceable; keep all bash usage strictly read-only.
+
+Strategy:
+1. Run `git diff` to see recent changes (if applicable)
+2. Read the modified files
+3. Check for bugs, security issues, code smells
+
+Output format:
+
+## Files Reviewed
+- `path/to/file.ts` (lines X-Y)
+
+## Critical (must fix)
+- `file.ts:42` - Issue description
+
+## Warnings (should fix)
+- `file.ts:100` - Issue description
+
+## Suggestions (consider)
+- `file.ts:150` - Improvement idea
+
+## Summary
+Overall assessment in 2-3 sentences.
+
+Be specific with file paths and line numbers.