feat: filesystem checkpoints and /rollback command

Automatic filesystem snapshots before destructive file operations,
with user-facing rollback.  Inspired by PR #559 (by @alireza78a).

Architecture:
- Shadow git repos at ~/.hermes/checkpoints/{hash}/ via GIT_DIR
- CheckpointManager: take/list/restore, turn-scoped dedup, pruning
- Transparent — the LLM never sees it, no tool schema, no tokens
- Once per turn — only first write_file/patch triggers a snapshot

Integration:
- Config: checkpoints.enabled + checkpoints.max_snapshots
- CLI flag: hermes --checkpoints
- Trigger: run_agent.py _execute_tool_calls() before write_file/patch
- /rollback slash command in CLI + gateway (list, restore by number)
- Pre-rollback snapshot auto-created on restore (undo the undo)

Safety:
- Never blocks file operations — all errors silently logged
- Skips root dir, home dir, dirs >50K files
- Disables gracefully when git not installed
- Shadow repo completely isolated from project git

Tests: 35 new tests, all passing (2798 total suite)
Docs: feature page, config reference, CLI commands reference

website/docs/user-guide/configuration.md

@@ -663,6 +663,16 @@ browser:
   record_sessions: false         # Auto-record browser sessions as WebM videos to ~/.hermes/browser_recordings/
 ```
 
+## Checkpoints
+
+Automatic filesystem snapshots before destructive file operations. See the [Checkpoints feature page](/docs/user-guide/features/checkpoints) for details.
+
+```yaml
+checkpoints:
+  enabled: false                 # Enable automatic checkpoints (also: hermes --checkpoints)
+  max_snapshots: 50              # Max checkpoints to keep per directory
+```
+
 ## Delegation
 
 Configure subagent behavior for the delegate tool: