docs: fix config examples, check commands, and small repo hygiene

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As of v0.3.0 (c73dc6c).

Summary

Bundle of small documentation/hygiene fixes:

1. README NOVA_CONFIG example has no effect as written. README.md (Configuration section) shows:

   NOVA_CONFIG=/path/to/config.toml

   As a standalone line this assigns a shell variable that Nova never sees. It should either be exported or shown as a command prefix:

   export NOVA_CONFIG=/path/to/config.toml
   # or
   NOVA_CONFIG=/path/to/config.toml nova check

   docs/README.ja.md should be updated to match.

2. CONTRIBUTING.md uses cargo-fmt / cargo-clippy binary names. These hyphenated forms work inside the Nix dev shell but the conventional invocations are cargo fmt --check and cargo clippy --all-targets -- -D warnings (which is also what CI runs). Using the standard form avoids confusing contributors who are not using nix develop.

3. .gitignore contains both .envrc and !.envrc. The two lines (from the toptal direnv template) cancel each other out, and .envrc is intentionally committed. Both lines can be removed.

4. SegmentCache::evict_oldest relies on a small fixed capacity. The O(n) scan is fine for the current capacity of 128, but a short comment near the implementation would make that assumption explicit for future maintainers.

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