feat(allowlist): add Mojo support

Sub-issue of #470.

Add support for Mojo source files to the review allowlist:
- Extensions: `.mojo`, `.🔥`
- No conventional test-file exclude pattern (Mojo tooling has not settled on one)
- A `mojo.md` review rule doc covering ownership/borrowing conventions (`borrowed`/`inout`/`owned`, transfer semantics, `__copyinit__`/`__moveinit__`/`__del__`), unsafe pointer and C/Python FFI interop, the `PythonObject` boundary, `parallelize`/`vectorize` concurrency hazards, and Mojo-specific performance anti-patterns
- Registration in `system_rules.json`

Will follow with a PR.

Start by inspecting the review allowlist and system_rules.json, then use the requested mojo.md rule document as the guide for Mojo-specific review concerns. Done means .mojo and .🔥 files are allowed, no conventional test-file exclusion is added, mojo.md covers the listed ownership, FFI, concurrency, and performance topics, and the rule is registered in system_rules.json.

Written by the indexing model from the issue text.