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

[hyperpolymath]

Fixes the pre-existing robot-repo-automaton build error noted (out of scope) in #298.

The bug

Detector::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:

let re = regex::Regex::new(regex_str).ok()?;     // string Regex
...
if let Ok(content) = std::fs::read(file_path) {  // Vec<u8>
    if re.is_match(&content) {                   // ❌ expects &str, got &Vec<u8>

So the crate never compiled, and the entire DetectionMethod::ContentMatch detection path (dispatched at detector.rs:149) was dead. CI didn't catch it because the rust CodeQL job runs build-mode none (buildless) — nothing in CI did a full cargo build/test of this crate.

The fix (one line)

Switch to std::fs::read_to_string. This both fixes the type error and implements the "skip non-UTF8" intent already written in the comment directly above the read (read_to_string returns Err on non-UTF8 bytes, which the existing if let Ok skips). It matches the sibling content scan in hypatia.rs:433-435, which already uses read_to_string + is_match(&content).

I considered regex::bytes::Regex instead, but that changes detection semantics and contradicts the stated non-UTF8-skipping intent — read_to_string is the smaller, intent-preserving fix.

Regression test

Added test_content_match_detects_and_skips_non_utf8 (this path had zero coverage since it never compiled): asserts a positive regex match is detected, a non-match yields nothing, and a non-UTF8 file is skipped without panicking.

Verification

• OPENSSL_NO_VENDOR=1 cargo build — clean.
• cargo test — 101 passed, 0 failed (was 100; +1 new test).
• cargo fmt --check — detector.rs is clean. (Pre-existing rustfmt drift in catalog.rs/confidence.rs is untouched and out of scope, consistent with Deletion-guard PR-body marker, BotId::Oikosbot, rhodibot link fix #298.)

Scope is detector.rs only (+58/−1, no file deletions).

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https://claude.ai/code/session_01RozeeLxpJsd3WWFngaZWz3

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