fix(security): prevent ReDoS in verdict-prose and continuation matchers (#4943)

Two py/polynomial-redos sinks ran regexes with two adjacent \s-matching
quantifiers over untrusted model text, backtracking O(n^2) when the tail failed
on a whitespace flood:

  - routes/skills_routes.py: the last-resort verdict-from-prose extractor used
    `["\'\s:]*\s*` — the class already matches \s, so the trailing \s* was a
    redundant second quantifier. Dropped it (extracted to a documented module
    constant _VERDICT_PROSE_RE); the matched text is identical, the scan linear.
  - src/agent_loop.py _EXPLICIT_CONTINUATION_RE: `\s*[.!?]*\s*$` put two \s*
    around `[.!?]*`. Rewrote as `\s*(?:[.!?]+\s*)?$` — same accepted tails (no
    two \s* adjacent), linear. Portable form (no possessive quantifiers).

Both verified output-equivalent to the originals across a fuzz corpus. Adds
tests/test_redos_verdict_continuation.py pinning the unchanged match sets and
bounding the flood inputs (old patterns took seconds at 40k whitespace chars).

routes/skills_routes.py

@@ -22,6 +22,16 @@ from core.middleware import require_admin
 
 logger = logging.getLogger(__name__)
 
+# Last-resort verdict extraction from a teacher/verifier model's prose (run when
+# JSON parsing fails). `["\'\s:]*` already consumes whitespace, so the original
+# trailing `\s*` made two adjacent \s-matching quantifiers that backtrack O(n^2)
+# on a `verdict` + whitespace flood in untrusted model output (CodeQL
+# py/polynomial-redos). Without it a single unbounded quantifier remains — the
+# matched text is identical, and the scan is linear.
+_VERDICT_PROSE_RE = re.compile(
+    r'verdict["\'\s:]*["\']?(pass|needs_work|fail|inconclusive)', re.I
+)
+
 
 class SkillAddRequest(BaseModel):
     # New schema (preferred)
@@ -196,7 +206,7 @@ async def _eval_skill_run(skill_md: str, task: str, transcript: str,
         # Last resort: pull the verdict keyword straight out of the prose so a
         # clearly-decided run isn't thrown away as "unparseable".
         if v not in _VERDICTS:
-            km = _re.search(r'verdict["\'\s:]*\s*["\']?(pass|needs_work|fail|inconclusive)', text, _re.I)
+            km = _VERDICT_PROSE_RE.search(text)
             if km:
                 v = km.group(1).lower()
                 if data is None: