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/prompts/implement-and-review.md

@@ -0,0 +1,10 @@
+---
+description: Worker implements, reviewer reviews, worker applies feedback
+---
+Use the subagent tool with the chain parameter to execute this workflow:
+
+1. First, use the "worker" agent to implement: $@
+2. Then, use the "reviewer" agent to review the implementation from the previous step (use {previous} placeholder)
+3. Finally, use the "worker" agent to apply the feedback from the review (use {previous} placeholder)
+
+Execute this as a chain, passing output between steps via {previous}.

packages/coding-agent/examples/extensions/subagent/prompts/implement.md

@@ -0,0 +1,10 @@
+---
+description: Full implementation workflow - scout gathers context, planner creates plan, worker implements
+---
+Use the subagent tool with the chain parameter to execute this workflow:
+
+1. First, use the "scout" agent to find all code relevant to: $@
+2. Then, use the "planner" agent to create an implementation plan for "$@" using the context from the previous step (use {previous} placeholder)
+3. Finally, use the "worker" agent to implement the plan from the previous step (use {previous} placeholder)
+
+Execute this as a chain, passing output between steps via {previous}.

packages/coding-agent/examples/extensions/subagent/prompts/scout-and-plan.md

@@ -0,0 +1,9 @@
+---
+description: Scout gathers context, planner creates implementation plan (no implementation)
+---
+Use the subagent tool with the chain parameter to execute this workflow:
+
+1. First, use the "scout" agent to find all code relevant to: $@
+2. Then, use the "planner" agent to create an implementation plan for "$@" using the context from the previous step (use {previous} placeholder)
+
+Execute this as a chain, passing output between steps via {previous}. Do NOT implement - just return the plan.