signals: restore the pre-port flag marker emoji

#903 inadvertently replaced the legacy FLAG_MARKER (U+1F6A9, '🚩') with
'[!]', which broke any downstream dashboard / alert that searches span
names for the flag emoji. Restores the original marker and updates the
#910 docs pass to match.

- crates/brightstaff/src/signals/analyzer.rs: FLAG_MARKER back to
  "\\u{1F6A9}" with a comment noting the backwards-compatibility
  reason so it doesn't drift again.
- docs/source/concepts/signals.rst and docs/source/guides/observability/
  tracing.rst: swap every '[!]' reference (subheading text, example
  span name, tip box, dashboard query hint) back to 🚩.

Verified: cargo test -p brightstaff --lib (162 passed, 1 ignored);
sphinx-build clean on both files; rendered HTML shows 🚩 in all
flag-marker references.

Made-with: Cursor

docs/source/guides/observability/tracing.rst

@@ -114,11 +114,11 @@ Signals act as early warning indicators embedded in your traces:
 
 **Visual Flag Markers**
 
-When concerning signals are detected (disengagement, execution failures / loops, stagnation > 2, or ``poor`` / ``severe`` quality), Plano automatically appends a ``[!]`` marker to the span's operation name. This makes problematic traces immediately visible in your tracing UI without requiring additional queries.
+When concerning signals are detected (disengagement, execution failures / loops, stagnation > 2, or ``poor`` / ``severe`` quality), Plano automatically appends a 🚩 marker to the span's operation name. This makes problematic traces immediately visible in your tracing UI without requiring additional queries.
 
 **Example Span with Signals**::
 
-    # Span name: "POST /v1/chat/completions gpt-4 [!]"
+    # Span name: "POST /v1/chat/completions gpt-4 🚩"
     # Standard LLM attributes:
     llm.model = "gpt-4"
     llm.usage.total_tokens = 225