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

crates/brightstaff/src/signals/analyzer.rs

@@ -21,9 +21,10 @@ use super::schemas::{
 use super::text_processing::NormalizedMessage;
 
 /// Marker appended to the span operation name when concerning signals are
-/// detected. Kept in sync with the previous implementation for backward
-/// compatibility with downstream consumers.
-pub const FLAG_MARKER: &str = "[!]";
+/// detected. The 🚩 emoji (U+1F6A9) matches the pre-port implementation so
+/// downstream consumers that search for flagged traces by span-name emoji
+/// keep working.
+pub const FLAG_MARKER: &str = "\u{1F6A9}";
 
 /// ShareGPT-shaped row used as the canonical input to the analyzer's
 /// detectors. `from` is one of `"human"`, `"gpt"`, `"function_call"`,