feat(diff): cross-surface /diff with staged/all/session modes

Widen the cherry-picked /diff base (#4839 by @SHL0MS) into one
cross-surface implementation, folding in the review feedback and the
best ideas from the two sibling PRs (#22703, #53527):

- tools/working_diff.py: shared git collection layer — unstaged
  (default), staged, and all (vs HEAD) modes; untracked files folded in
  via `git diff --no-index` so new files appear as additions (Codex
  /diff parity); shlex-split arguments preserve quoted paths.
- CLI: handler moved to hermes_cli/cli_commands_mixin.py per the
  current god-file decomposition (dispatch stays in cli.py), renders
  through the rich console with a 400-line terminal-flood guard.
- Gateway: _handle_diff_command in gateway/slash_commands.py + dispatch
  in gateway/run.py; fenced ```diff output truncated to 60 lines /
  3000 chars before the platform senders apply their own per-platform
  message clamps (tool-progress-style layered truncation). Localized
  strings in all 17 locale catalogs.
- /diff session (from #53527): cumulative checkpoint-baseline diff of
  everything Hermes changed, via new CheckpointManager.session_diff();
  docstring records the retained-baseline approximation caveat from
  review. Works on both surfaces; degrades with an actionable message
  when checkpoints are off.
- Slack: /diff routed via /hermes diff (50-slash cap; keeps
  telegram-parity test green and /version native).
- Registry: cross-surface CommandDef with staged|all|session
  subcommands; docs: slash-commands reference (CLI + gateway tables +
  both-surfaces list) and hermes-agent skill reference.
- Tests: tests/tools/test_working_diff.py (real git repos),
  tests/hermes_cli/test_diff_command.py (real git + stubbed checkpoint
  manager), tests/gateway/test_diff_command.py (end-to-end handler,
  real checkpoint store), TestSessionDiff in
  tests/tools/test_checkpoint_manager.py.

Salvaged from the /diff PR cluster #4839 + #22703 + #53527.

Co-authored-by: Ninso112 <ninso112@proton.me>
Co-authored-by: Harshkamdar67 <harshkamdar67@gmail.com>

locales/ga.yaml

@@ -283,6 +283,11 @@ Future messages in this room will use that transcript until `/reset` or another
     restored:              "✅ Aischurtha go seicphointe {hash}: {reason}\nSábháladh roghchóip réamh-rollback go huathoibríoch."
     restore_failed:        "❌ {error}"
 
+  diff:
+    not_enabled:           "Níl seicphointí cumasaithe, mar sin níl aon bhonnlíne seisiúin ann.\nCumasaigh in config.yaml:\n```\ncheckpoints:\n  enabled: true\n```\nOibríonn /diff simplí fós — úsáideann sé git go díreach."
+    no_changes:            "Gan athruithe."
+    failed:                "{error}"
+
   set_home:
     save_failed:           "Theip ar shábháil chainéil bhaile: {error}"
     success:               "✅ Cainéal baile socraithe go **{name}** (ID: {chat_id}).\nSeachadfar tascanna cron agus teachtaireachtaí trasardáin anseo."