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

test.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+set -e
+
+AUTH_FILE="$HOME/.pi/agent/auth.json"
+AUTH_BACKUP="$HOME/.pi/agent/auth.json.bak"
+
+# Restore auth.json on exit (success or failure)
+cleanup() {
+    if [[ -f "$AUTH_BACKUP" ]]; then
+        mv "$AUTH_BACKUP" "$AUTH_FILE"
+        echo "Restored auth.json"
+    fi
+}
+trap cleanup EXIT
+
+# Move auth.json out of the way
+if [[ -f "$AUTH_FILE" ]]; then
+    mv "$AUTH_FILE" "$AUTH_BACKUP"
+    echo "Moved auth.json to backup"
+fi
+
+# Skip local LLM tests (ollama, lmstudio)
+export PI_NO_LOCAL_LLM=1
+
+# Unset API keys (see packages/ai/src/stream.ts getEnvApiKey)
+unset ANTHROPIC_API_KEY
+unset ANTHROPIC_OAUTH_TOKEN
+unset OPENAI_API_KEY
+unset GEMINI_API_KEY
+unset GROQ_API_KEY
+unset CEREBRAS_API_KEY
+unset XAI_API_KEY
+unset OPENROUTER_API_KEY
+unset ZAI_API_KEY
+unset MISTRAL_API_KEY
+unset COPILOT_GITHUB_TOKEN
+unset GH_TOKEN
+unset GITHUB_TOKEN
+
+echo "Running tests without API keys..."
+npm test