fix(robot-repo-automaton): make ContentMatch detection compile

`detect_content_match` read file contents with `std::fs::read` (→ `Vec<u8>`)
and passed `&content` to a string `regex::Regex::is_match`, which wants
`&str`. The crate therefore never compiled and the whole
`DetectionMethod::ContentMatch` detection path was dead code.

Switch to `std::fs::read_to_string`. This fixes the type error and also
implements the "skip non-UTF8" intent already stated in the comment just
above (`read_to_string` returns `Err` on non-UTF8 bytes, which the
`if let Ok` then skips), matching the sibling content scan in
`hypatia.rs`.

Add a regression test for the previously-uncompilable path: a positive
regex match, a non-match, and a non-UTF8 file that must be skipped
without panicking. `cargo build` is clean and all 101 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RozeeLxpJsd3WWFngaZWz3

Author: claude

robot-repo-automaton/src/detector.rs

@@ -297,7 +297,9 @@ impl Detector {
                     }
                 }
 
-                if let Ok(content) = std::fs::read(file_path) {
+                // read_to_string returns Err on non-UTF8 bytes, so the
+                // `if let Ok` here also implements the "skip non-UTF8" intent.
+                if let Ok(content) = std::fs::read_to_string(file_path) {
                     if re.is_match(&content) {
                         affected.push(file_path.clone());
                     }
@@ -377,6 +379,8 @@ impl Detector {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::catalog::{Detection, Fix, FixAction};
+    use std::collections::HashMap;
     use tempfile::TempDir;
 
     #[test]
@@ -404,4 +408,57 @@ mod tests {
         assert!(detector.file_exists(".github/workflows/ci.yml"));
         assert!(!detector.file_exists(".github/workflows/nonexistent.yml"));
     }
+
+    fn content_match_error(condition: &str, files: Vec<String>) -> ErrorType {
+        ErrorType {
+            id: "TEST-CONTENT".to_string(),
+            name: "content-match regression".to_string(),
+            severity: Severity::Medium,
+            category: "test".to_string(),
+            description: "regression coverage for detect_content_match".to_string(),
+            detection: Detection {
+                method: DetectionMethod::ContentMatch,
+                files,
+                condition: Some(condition.to_string()),
+                extension_map: HashMap::new(),
+            },
+            affected_repos: vec![],
+            fix: Fix {
+                action: FixAction::Modify,
+                target: String::new(),
+                reason: None,
+                modification: None,
+                fallback: None,
+            },
+            commit_message: "test".to_string(),
+        }
+    }
+
+    #[test]
+    fn test_content_match_detects_and_skips_non_utf8() {
+        let temp = TempDir::new().unwrap();
+        std::fs::write(temp.path().join("hit.txt"), "contains believe_me here").unwrap();
+        std::fs::write(temp.path().join("miss.txt"), "nothing to see").unwrap();
+        // Invalid UTF-8: detect_content_match must skip this (not panic) — the
+        // bug fix relies on std::fs::read_to_string returning Err here.
+        std::fs::write(temp.path().join("blob.bin"), [0xff, 0xfe, 0x62, 0x6d]).unwrap();
+
+        let detector = Detector::new(temp.path().to_path_buf()).unwrap();
+
+        // Positive: regex hits hit.txt; the non-UTF8 blob is skipped cleanly.
+        assert!(
+            detector
+                .detect(&content_match_error("believe_me", vec!["*".to_string()]))
+                .is_some(),
+            "should detect the file whose contents match the regex"
+        );
+
+        // Negative: a token present in no valid-UTF8 file yields no issue.
+        assert!(
+            detector
+                .detect(&content_match_error("no_such_token", vec!["*".to_string()]))
+                .is_none(),
+            "should report nothing when no file matches"
+        );
+    }
 }