feat(gateway): opt-in runtime-metadata footer on final replies (#17026)

Append a compact 'model · 68% · ~/projects/hermes' footer to the FINAL
message of each turn, disabled by default (display.runtime_footer.enabled).
Answers the Telegram-side parity ask: runtime context that the CLI status
bar already shows is now available in messaging replies when enabled.

Wiring:
- gateway/runtime_footer.py: resolve_footer_config + format_runtime_footer +
  build_footer_line. Pure-function renderer; per-platform overrides under
  display.platforms.<platform>.runtime_footer.
- gateway/run.py: appends footer to response right after reasoning prepend
  so it lands only on the final message (never tool progress or streaming
  chunks). When streaming already delivered the body (already_sent), the
  footer is sent as a small trailing message instead.
- agent_result now exposes context_length alongside last_prompt_tokens so
  the footer can compute the pct; both gateway return paths updated.
- /footer [on|off|status] slash command, wired in CLI (cli.py) and gateway
  (gateway/run.py both running-agent bypass and main dispatch). Global
  toggle only; per-platform overrides via config.yaml.

Graceful degradation:
- Missing context_length (unknown model) → pct field silently dropped
  (no '?%' artifact).
- Empty final_response → no footer appended.
- Unknown field names in config → silently ignored.

Tests: 25-case unit suite (tests/gateway/test_runtime_footer.py) plus E2E
harness covering streaming vs non-streaming branches, per-platform override,
and the exact argument contract gateway/run.py uses.

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

hermes_cli/config.py

@@ -707,6 +707,14 @@ DEFAULT_CONFIG = {
         "tool_progress_overrides": {},  # DEPRECATED — use display.platforms instead
         "tool_preview_length": 0,  # Max chars for tool call previews (0 = no limit, show full paths/commands)
         "platforms": {},  # Per-platform display overrides: {"telegram": {"tool_progress": "all"}, "slack": {"tool_progress": "off"}}
+        # Gateway runtime-metadata footer appended to the FINAL message of a turn
+        # (disabled by default to keep replies minimal). When enabled, renders
+        # e.g. `model · 68% · ~/projects/hermes`. Per-platform overrides go under
+        # display.platforms.<platform>.runtime_footer.
+        "runtime_footer": {
+            "enabled": False,
+            "fields": ["model", "context_pct", "cwd"],  # Order shown; drop any to hide
+        },
     },
 
     # Web dashboard settings