fix(gateway): make /model session-scoped by default on messaging platforms

[liuhao1024]

What does this PR do?

Changes /model behavior on messaging platforms (gateway) from persisting to global config.yaml by default to session-scoped by default.

Previously, /model on gateway would follow CLI behavior via the model.persist_switch_by_default config setting, which defaults to True. This meant switching models in one chat would write the change to the global config.yaml, affecting all other sessions — unintended cross-session pollution.

Messaging platform sessions are typically ephemeral and per-chat, so this PR changes the default to session-scoped: the switch applies to the current chat only, survives gateway restart via the session DB, but does not persist to disk unless --global is explicitly passed.

This aligns gateway /model behavior with user expectations for messaging platform UX, while CLI behavior (persistence by default) remains unchanged.

Related Issue

Fixes #63083

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/slash_commands.py: Changed persist_global logic to default to False on messaging platforms unless --global is explicitly passed or --session is explicitly passed. Added comment explaining the rationale and reference to /model writes to global config.yaml without --global flag on cross-provider switch attempts #63083.
• tests/gateway/test_model_picker_persist.py: Updated test_picker_tap_persists_by_default → test_picker_tap_does_not_persist_by_default to reflect the new behavior (session-scoped by default). Added new test test_picker_tap_with_global_flag_persists to verify --global flag works correctly.
• tests/gateway/test_model_command_flat_string_config.py: Updated test_model_no_flag_persists_by_default → test_model_no_flag_does_not_persist_by_default to reflect the new behavior.

How to Test

1. Run gateway on a messaging platform (Telegram/Discord)
2. Start a session with model A
3. In a chat, run /model gpt-5.5 (no flags)
4. Verify: the session switches to gpt-5.5 and shows "session only" hint
5. Check ~/.hermes/config.yaml — should still have model A
6. In the same chat, run /model claude-sonnet-4.6 --global
7. Verify: the session switches to claude-sonnet-4.6 and shows "saved to global config" hint
8. Check ~/.hermes/config.yaml — should now have claude-sonnet-4.6
9. Run pytest tests/gateway/test_model_picker_persist.py tests/gateway/test_model_command_flat_string_config.py -q — all tests pass

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.4.1

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

[alt-glitch]

This was generated by AI during triage.

Duplicate of #61488 (liuhao... earlier, still open) — diff-compared: both edit gateway/slash_commands.py at the same site in _handle_model_command (right after parse_model_flags) with the identical mechanism: default /model to session-scope on messaging (non-LOCAL) platforms unless --global is explicitly passed, to stop picker taps from writing the global config.yaml. Both also update the same test_model_no_flag_persists_by_default test. #61488 (2026-07-09) predates this PR (2026-07-12) and is the canonical fix; this one cites #63083 while #61488 cites #61458, but the code-site and mechanism are the same. Same gateway /model session-scope cluster as #62827 / #59493.

[liuhao1024]

This PR is a duplicate of #$61488.

Both PRs fix the same issue (messaging platform /model command defaulting to global config) by modifying the same code path (_handle_model_command in gateway/slash_commands.py).

PR #$61488 (created 2026-07-09) is the earlier submission with a more explicit implementation (checking source.platform type).

Closing this PR in favor of the canonical earlier submission.