chore(tools): Support per-pattern regex mode in fs_modify_file

[JeanMertz]

Patterns passed to fs_modify_file can now set their own regex flag, overriding the call-level replace_using_regex. This lets a single call mix literal and regex patterns instead of forcing all patterns into the same mode:

{
  "path": "src/lib.rs",
  "patterns": [
    {"old": "stream().await", "new": "stream()"},
    {"old": "fn (\w+)_old", "new": "fn ${1}_new", "regex": true}
  ],
  "replace_using_regex": false
}

Fixes two related bugs in regex mode. A regex that compiled but matched nothing was previously reported as applied even though no replacement happened; it is now reported as not found alongside literal-mode misses. A regex that failed to compile silently skipped the pattern with no feedback; it now surfaces as an "Invalid regex patterns" section in the report, separate from "Patterns not found", so callers can tell a typo in the regex apart from a pattern that simply is not present.

The find_blocked_regex_patterns guard against overly-broad patterns (e.g. bare .*) now only applies to patterns actually running in regex mode, respecting the same per-pattern override. Tool docs and the JSON schema are updated to describe the new regex field and to warn that parentheses are capture groups, not literal characters, in regex mode.