feat(diff): /diff — review everything Hermes changed this session

[Harshkamdar67]

What does this PR do?

Adds a /diff command that shows everything Hermes changed in the working directory this session - the cumulative diff from the earliest retained checkpoint (the pre-edit baseline) to the current working tree. It answers "what did you actually change?" before committing, in one view.

This is the checkpoint/session-scoped approach requested in #4865 (approach #2 - "diff against checkpoint 0 (session start)", and the issue's Update note: "aggregate existing per-edit checkpoint diffs into a session-level summary").

Relationship to existing /diff PRs (#4839, #22703)

I found two open /diff PRs while checking for duplicates. Both are thin wrappers around git diff in the cwd, and neither addresses what #4865 actually asks for:

	#4839 / #22703	This PR
Mechanism	raw git diff / git diff --cached	diff vs the earliest checkpoint (session baseline)
Requires the project to be a git repo	Yes - bails with "Not a git repository"	No - uses Hermes's checkpoint shadow store
Scoped to what Hermes changed	No - shows all your working-tree edits too	Yes - only changes since the session's first checkpoint
Surfaces	CLI only	CLI + messaging gateway + TUI

If maintainers prefer one of the raw-git approaches, I'm happy to close this - but it's the only one of the three that solves the "what did the agent change this session" UX in #4865, and it works in non-git project directories where the others don't.

Related Issue

Fixes #4865

Type of Change

• New feature (non-breaking change that adds functionality)

Changes Made

• tools/checkpoint_manager.py - new CheckpointManager.session_diff() that diffs the earliest retained checkpoint against the working tree (reuses the existing .diff() staging machinery; returns {success, stat, diff, empty}).
• hermes_cli/commands.py - register /diff [--stat] in COMMAND_REGISTRY.
• hermes_cli/cli_commands_mixin.py + cli.py - CLI handler _handle_diff_command and dispatch.
• gateway/slash_commands.py + gateway/run.py + locales/en.yaml - gateway handler and dispatch (other locales fall back to English via t()).
• tui_gateway/server.py - diff.session JSON-RPC method (counterpart to rollback.diff).
• Docs: website/docs/reference/slash-commands.md, agent SKILL.md command list.

How to Test

1. Run with checkpoints on: hermes --checkpoints
2. Ask Hermes to edit a couple of files.
3. Run /diff for the cumulative diff of all its edits this session; /diff --stat for the summary only.
4. Works even if the project directory is not a git repo.

Automated:

• pytest tests/tools/test_checkpoint_manager.py::TestSessionDiff -q (session_diff unit tests)
• pytest tests/hermes_cli/test_diff_command.py -q (CLI handler)
• pytest tests/gateway/test_diff_command.py -q (gateway E2E - real git + checkpoint store)
• pytest tests/test_tui_gateway_server.py -k diff_session -q (TUI RPC)

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate (see "Relationship to existing PRs" above - same /diff name, materially different approach)
• My PR contains only changes related to this feature
• I've added tests for my changes
• I've tested on my platform: Windows 11

Documentation & Housekeeping

• I've updated relevant documentation (docs/, SKILL.md command list)
• No new config keys (reuses the existing checkpoints config) - cli-config.yaml.example N/A
• Cross-platform: pure git/subprocess via the existing checkpoint engine; ruff + scripts/check-windows-footguns.py clean

[alt-glitch]

This was generated by AI during triage.

Competing /diff implementation alongside open #4839 and #22703 (both raw git diff wrappers that require a git repo). This PR instead diffs against the session-start checkpoint via the checkpoint shadow store (Fixes #4865 approach #2) and works in non-git directories. Flagging the three-PR cluster so a maintainer can pick the mechanism.

[tonydwb]

Code Review Summary

Verdict: Approved

New /diff command that shows the cumulative diff from the earliest retained checkpoint to the current working tree. Complements /rollback diff <N> (single-checkpoint preview).

Looks Good

• Clean implementation across CLI, gateway, and slash_commands
• --stat flag for summary-only output
• Diff truncation at 60 lines with helpful message
• Gateway version properly loads checkpoint config from config.yaml
• Integration with existing CheckpointManager.session_diff()
• Localized strings used for user-facing messages

14 files touched, all related to the same feature. Well-scoped.

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Reviewed by Hermes Agent

[teknium1]

Thanks for pursuing the checkpoint-backed variant and for covering the CLI, gateway, and backend RPC paths.

Problems

• tools/checkpoint_manager.py's proposed session_diff() selects the oldest retained item from list_checkpoints(). On current main, that list is a persistent per-project ref (tools/checkpoint_manager.py:690-729) and pruning rewrites it independently of sessions (tools/checkpoint_manager.py:1053-1118). It therefore cannot guarantee a current-session baseline: it may include prior-session changes or lose the real session-start state.
• The proposed diff.session RPC is not wired into the TUI. This PR changes no ui-tui/src files. Current ui-tui/src/app/createSlashHandler.ts:40-148 routes unregistered commands to slash.exec, and that worker creates a separate HermesCLI (tui_gateway/slash_worker.py:155-168) instead of invoking the live-session RPC.

Suggested changes

• Track a baseline explicitly by live session and working directory, then add regressions for consecutive sessions and retention pruning.
• Add a native TUI /diff route that calls diff.session, plus an end-to-end slash-handler test.

This is an automated hermes-sweeper review.

[teknium1]

The session-scoped diff mode from this PR landed via #72240 (now merged) — /diff session shows only files Hermes touched this session, credited in the unifying PR. Thanks!