feat(allowlist): add Mojo support

[wakqasahmed]

Summary

• Add .mojo and .🔥 to the supported file types allowlist
• No default exclude pattern added — Mojo tooling has not settled on a conventional test-directory layout yet, so allowed_ext_test.go documents this explicitly (matching the existing Prisma/Shader precedent)
• Add internal/config/rules/rule_docs/mojo.md, a Mojo review rule doc covering ownership/borrowing conventions (borrowed/inout/owned argument mismatches, unintended transfers with ^, __copyinit__/__moveinit__/__del__ correctness), value semantics and struct design, unsafe pointer and C/Python FFI interop, the PythonObject interop boundary, parallelize/vectorize concurrency hazards, Mojo-specific performance anti-patterns, and security-sensitive raw-pointer/deserialization concerns
• Register **/*.{mojo,🔥} -> mojo.md in system_rules.json
• Add test coverage: allowed-extension matching (both extensions, including the fire-emoji extension), non-exclusion of Mojo files, and rule resolution

Part of the language-allowlist expansion tracked in #470.

Closes #976

Test plan

• go test ./internal/config/allowlist/... -run 'TestIsAllowedExt|TestIsExcludedPath' -v — PASS
• go test ./internal/config/rules/... -run TestResolve_DefaultRules -v — PASS
• gofmt -s -l . — clean
• go vet ./internal/config/... — clean
• go run scripts/verify-english-only.go — clean (423 files scanned, no unapproved non-English text)

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 2 selected item(s).

[wu21-web]

Site documentation updates missing, update at pages/src/content/docs/<locale>/review-rules.md.

[wakqasahmed]

@wu21-web Thanks for the catch — docs added in the latest commit. Updated pages/src/content/docs/{en,ja,ru,zh}/review-rules.md with a row for the new **/*.{mojo,🔥} → mojo.md mapping, mirroring the format used for the Elm entry (#978).

Note: the ja/ru/zh translations are my best effort — please have a native speaker double-check the phrasing before merge.

[wu21-web]

@wakqasahmed Same for here, merge conflicts.

[wakqasahmed]

@wu21-web Resolved — rebased onto latest main and merged, fixing the conflicts in supported_file_types.json, system_rules.json, the two allowlist/rules test files, and all four locale review-rules.md docs (they were conflicting with the recently-merged jsonnet/zig/thrift/capnp entries). Should be clean to review now.