perf: fix O(n²) catastrophic backtracking in redact regex + reorder file read guard

Two pre-existing issues causing test_file_read_guards timeouts on CI:

1. agent/redact.py: _ENV_ASSIGN_RE used unbounded [A-Z_]* with
   IGNORECASE, matching any letter/underscore to end-of-string at
   each position → O(n²) backtracking on 100K+ char inputs.
   Bounded to {0,50} since env var names are never that long.

2. tools/file_tools.py: redact_sensitive_text() ran BEFORE the
   character-count guard, so oversized content (that would be rejected
   anyway) went through the expensive regex first. Reordered to check
   size limit before redaction.

agent/redact.py

@@ -53,7 +53,7 @@ _PREFIX_PATTERNS = [
 # ENV assignment patterns: KEY=value where KEY contains a secret-like name
 _SECRET_ENV_NAMES = r"(?:API_?KEY|TOKEN|SECRET|PASSWORD|PASSWD|CREDENTIAL|AUTH)"
 _ENV_ASSIGN_RE = re.compile(
-    rf"([A-Z_]*{_SECRET_ENV_NAMES}[A-Z_]*)\s*=\s*(['\"]?)(\S+)\2",
+    rf"([A-Z_]{{0,50}}{_SECRET_ENV_NAMES}[A-Z_]{{0,50}})\s*=\s*(['\"]?)(\S+)\2",
     re.IGNORECASE,
 )