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/reference/cli-commands.md

@@ -24,6 +24,7 @@ These are commands you run from your shell.
 | `hermes chat --toolsets "web,terminal"` / `-t` | Use specific toolsets |
 | `hermes chat --verbose` | Enable verbose/debug output |
 | `hermes --worktree` / `-w` | Start in an isolated git worktree (for parallel agents) |
+| `hermes --checkpoints` | Enable filesystem checkpoints before destructive file operations |
 
 ### Provider & Model Management
 
@@ -202,6 +203,8 @@ These work in messaging platforms (Telegram, Discord, Slack, WhatsApp) but not t
 | `/sethome` | Set this chat as the home channel |
 | `/status` | Show session info |
 | `/reload-mcp` | Reload MCP servers from config |
+| `/rollback` | List filesystem checkpoints for the current directory |
+| `/rollback <N>` | Restore files to checkpoint #N |
 | `/update` | Update Hermes Agent to the latest version |
 
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