fix(skills): string-aware trailing comma stripping for JSONC

[itxaiohanglover]

Fixes #553

Problem

The regex r',(\s*[}\]])' in skills.py:351 strips trailing commas before } or ], but does not track JSON string boundaries. If a string value contains a comma followed by } or ] on the next line, the regex incorrectly removes the comma inside the string, corrupting the JSON.

Example breakage:

{
  "mcp": {
    "my-server": {
      "description": "foo, bar",
    }
  }
}

The comma in "foo, bar" gets removed → "foo bar" — silently corrupting user config.

What changed

Replaced the regex with _strip_trailing_commas_jsonc() — a minimal stateful parser that:

1. Tracks in_string state (toggles on " outside of escape sequences)
2. Only strips commas that are followed by } or ] outside of strings
3. Handles escape sequences (\", \\) correctly

No new dependencies. No existing comments modified.

Validation

• The existing json.loads(stripped) call on line 353 works unchanged
• Strings containing ,} or ,] patterns are preserved
• Trailing commas outside strings are still stripped correctly