Add config-based close hooks (bd-g4b4)

Implements on_close hooks in .beads/config.yaml for automation and
notifications. Hooks receive issue data via environment variables
(BEAD_ID, BEAD_TITLE, BEAD_TYPE, BEAD_PRIORITY, BEAD_CLOSE_REASON)
and run via sh -c.

Changes:
- internal/config: Add HookEntry type and GetCloseHooks()
- internal/hooks: Add RunConfigCloseHooks() for executing config hooks
- cmd/bd: Call RunConfigCloseHooks after successful close
- docs/CONFIG.md: Document hooks configuration with examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/CONFIG.md

@@ -104,6 +104,73 @@ external_projects:
   gastown: /path/to/gastown
 ```
 
+### Hooks Configuration
+
+bd supports config-based hooks for automation and notifications. Currently, close hooks are implemented.
+
+#### Close Hooks
+
+Close hooks run after an issue is successfully closed via `bd close`. They execute synchronously but failures are logged as warnings and don't block the close operation.
+
+**Configuration:**
+
+```yaml
+# .beads/config.yaml
+hooks:
+  on_close:
+    - name: show-next
+      command: bd ready --limit 1
+    - name: context-check
+      command: echo "Issue $BEAD_ID closed. Check context if nearing limit."
+    - command: notify-team.sh  # name is optional
+```
+
+**Environment Variables:**
+
+Hook commands receive issue data via environment variables:
+
+| Variable | Description |
+|----------|-------------|
+| `BEAD_ID` | Issue ID (e.g., `bd-abc1`) |
+| `BEAD_TITLE` | Issue title |
+| `BEAD_TYPE` | Issue type (`task`, `bug`, `feature`, etc.) |
+| `BEAD_PRIORITY` | Priority (0-4) |
+| `BEAD_CLOSE_REASON` | Close reason if provided |
+
+**Example Use Cases:**
+
+1. **Show next work item:**
+   ```yaml
+   hooks:
+     on_close:
+       - name: next-task
+         command: bd ready --limit 1
+   ```
+
+2. **Context check reminder:**
+   ```yaml
+   hooks:
+     on_close:
+       - name: context-check
+         command: |
+           echo "Issue $BEAD_ID ($BEAD_TITLE) closed."
+           echo "Priority was P$BEAD_PRIORITY. Reason: $BEAD_CLOSE_REASON"
+   ```
+
+3. **Integration with external tools:**
+   ```yaml
+   hooks:
+     on_close:
+       - name: slack-notify
+         command: curl -X POST "$SLACK_WEBHOOK" -d "{\"text\":\"Closed: $BEAD_ID - $BEAD_TITLE\"}"
+   ```
+
+**Notes:**
+- Hooks have a 10-second timeout
+- Hook failures log warnings but don't fail the close operation
+- Commands run via `sh -c`, so shell features like pipes and redirects work
+- Both script-based hooks (`.beads/hooks/on_close`) and config-based hooks run
+
 ### Why Two Systems?
 
 **Tool settings (Viper)** are user preferences: