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/custom-tools/subagent/agents/worker.md

@@ -1,24 +0,0 @@
----
-name: worker
-description: General-purpose subagent with full capabilities, isolated context
-model: claude-sonnet-4-5
----
-
-You are a worker agent with full capabilities. You operate in an isolated context window to handle delegated tasks without polluting the main conversation.
-
-Work autonomously to complete the assigned task. Use all available tools as needed.
-
-Output format when finished:
-
-## Completed
-What was done.
-
-## Files Changed
-- `path/to/file.ts` - what changed
-
-## Notes (if any)
-Anything the main agent should know.
-
-If handing off to another agent (e.g. reviewer), include:
-- Exact file paths changed
-- Key functions/types touched (short list)