fix(security): match grep's rg sensitive-file exclusions case-insensitively (#5189)

The grep tool's ripgrep fast-path excluded deny-listed key files with
`--glob "!*<pat>*"` for each entry in _SENSITIVE_FILE_PATTERNS. ripgrep's
--glob is case-sensitive, so on a case-insensitive filesystem (Windows,
default macOS) a key stored under a case variant of its name (ID_RSA,
Known_Hosts, Authorized_Keys) is the same file on disk but slips past the
lowercase exclusion, and ripgrep returns its contents. Those names are
non-dotfiles, so ripgrep's default hidden-file skipping does not cover them
either. The Python fallback already blocks them via the case-folded
_is_sensitive_path (#5097), so the two paths disagreed.

Switch the sensitive-pattern exclusions to --iglob so they match
case-insensitively, mirroring _is_sensitive_path. Add a regression test
that seeds ID_RSA and Known_Hosts and asserts grep returns ordinary
matches but not the key contents.

src/agent_tools/filesystem_tools.py

@@ -424,8 +424,12 @@ class GrepTool:
                     cmd.append("--ignore-case")
                 if glob_pat:
                     cmd += ["--glob", glob_pat]
+                # --iglob (not --glob) so the exclusion is case-insensitive:
+                # on a case-insensitive filesystem "ID_RSA"/"Known_Hosts"
+                # resolve to the same secret as their lowercase forms, and the
+                # Python fallback below already folds case via _is_sensitive_path.
                 for _pat in _SENSITIVE_FILE_PATTERNS:
-                    cmd += ["--glob", f"!*{_pat}*"]
+                    cmd += ["--iglob", f"!*{_pat}*"]
                 for _d in _CODENAV_SKIP_DIRS:
                     cmd += ["--glob", f"!**/{_d}/**"]
                 cmd += ["--regexp", pattern, root]