docs: add pre-exec hooks documentation

Covers hook discovery, protocol, writing hooks, and example usage.

docs/tools/pre-exec-hooks.md

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+# Pre-Exec Hooks
+
+Pre-exec hooks let workspace-level scripts intercept and approve/deny shell commands before they run. This provides a safety net for AI agent operations — preventing pushes to protected branches, writes to production databases, or dangerous file operations.
+
+## How It Works
+
+```
+Agent runs: git push origin develop
+         ↓
+Clawdbot exec tool → runs pre-exec hooks
+         ↓
+~/.clawdbot/hooks/safe-git.sh → receives command as JSON
+         ↓
+Hook outputs: {"decision": "deny", "reason": "🚫 Protected branch"}
+         ↓
+Exec tool throws error instead of running command
+```
+
+## Hook Discovery
+
+Hooks are discovered from these directories (in order):
+
+1. `<workspace>/.clawdbot/hooks/` (preferred)
+2. `<workspace>/hooks/` (fallback)
+
+Any executable shell script in these directories is treated as a hook.
+
+## Hook Protocol
+
+### Input (JSON on stdin)
+
+```json
+{
+  "tool_name": "exec",
+  "tool_input": {
+    "command": "git push origin main",
+    "workdir": "/path/to/workspace",
+    "env": {}
+  }
+}
+```
+
+### Output (JSON on stdout)
+
+```json
+{
+  "decision": "approve",
+  "reason": "optional message"
+}
+```
+
+Or to deny:
+
+```json
+{
+  "decision": "deny", 
+  "reason": "🚫 Pushing to protected branches is blocked."
+}
+```
+
+## Writing a Hook
+
+Create an executable script in `.clawdbot/hooks/`:
+
+```bash
+#!/bin/bash
+# .clawdbot/hooks/block-sudo.sh
+
+INPUT=$(cat)
+COMMAND=$(echo "$INPUT" | jq -r '.tool_input.command // empty')
+
+if echo "$COMMAND" | grep -qE '(^|\s)sudo\s'; then
+  echo '{"decision": "deny", "reason": "🚫 sudo is not allowed."}'
+  exit 0
+fi
+
+echo '{"decision": "approve"}'
+```
+
+Make it executable:
+
+```bash
+chmod +x .clawdbot/hooks/block-sudo.sh
+```
+
+## Example Hooks
+
+Clawdbot includes example hooks in `examples/pre-exec-hooks/`:
+
+### safe-git.sh
+
+Blocks:
+- Force pushes (`--force`, `-f`)
+- Pushes to protected branches (main, develop, staging, production)
+- Remote modifications (`git remote add/remove/set-url`)
+
+### safe-db.sh
+
+Blocks:
+- Non-SELECT operations on remote databases
+- Migrations/seeds targeting staging/production environments
+
+### safe-rm.sh
+
+Blocks:
+- `rm -rf /`
+- `rm` on home/system directories
+- `rm -rf *` (wildcard deletion)
+
+## Behavior
+
+- **Sequential execution**: Hooks run in alphabetical order
+- **Short-circuit**: First "deny" stops execution
+- **Fail-open**: Timeouts and errors default to "approve"
+- **Timeout**: 10 seconds per hook (configurable)
+
+## Environment Variables
+
+Hooks receive these environment variables:
+
+| Variable | Description |
+|----------|-------------|
+| `CLAWDBOT_HOOK_NAME` | Name of the current hook |
+| `CLAWDBOT_TOOL_NAME` | Tool being invoked (`exec` or `Bash`) |
+
+## Tips
+
+1. **Use jq** for parsing JSON input
+2. **Keep hooks fast** — they run on every command
+3. **Log to stderr** — only stdout is parsed
+4. **Test locally** before deploying
+
+```bash
+# Test a hook manually
+echo '{"tool_name":"exec","tool_input":{"command":"git push origin main"}}' | .clawdbot/hooks/safe-git.sh
+```
+
+## Related
+
+- [Exec Tool](exec.md) — Shell command execution
+- [Exec Approvals](exec-approvals.md) — Allowlist-based command approval