feat(gateway): add runtime footer turn count

[w3rdist-creator]

Summary

• Adds optional runtime footer turn_count field, rendered as N/limit
• Resolves turn_limit from global or per-platform runtime footer config, defaulting to 100
• Passes computed gateway user-turn count into footer rendering

Test Plan

• python -m pytest tests/gateway/test_runtime_footer.py -q -o 'addopts='

[teknium1]

Thanks for the focused runtime-footer addition. The requested capability is still absent on current main: gateway/runtime_footer.py:105-118 recognizes only the existing three fields, and gateway/run.py:11827-11834 supplies no turn count.

Problems

• The new config surface needs current default-config and docs coverage. hermes_cli/config.py:1922-1925 and website/docs/user-guide/configuration.md:1476-1483 list only model, context_pct, and cwd.
• /footer on builds its preview through format_runtime_footer without a count (gateway/slash_commands.py:3073-3078), so a fields: [turn_count] configuration would produce no preview.
• The gateway target has moved from the PR's old line 9408 to current gateway/run.py:11820-11839; this needs manual reapplication and a current-path integration test.

Suggested changes

• Salvage the call at the current footer site, cover the final-response path, document turn_count/turn_limit, and define a preview value or omission rule for turn count.

Automated hermes-sweeper review.

[teknium1]

Please also handle the /footer on preview path: it calls format_runtime_footer without turn_count (gateway/slash_commands.py:3073-3078), so configuring only this new field would render an empty preview.