feat: add corpus subcommand to harper-cli

[majiayu000]

Summary

Adds a corpus subcommand to harper-cli that reads a file of marked sentences and detects false positives and negatives.

• Lines prefixed with ✅ are expected to produce no lints (pass)
• Lines prefixed with ❌ are expected to produce one or more lints (fail)
• Supports optional --rule flag to restrict testing to a single linter
• Supports inline `RuleName` syntax per line (as described in the issue)
• Outputs colored [PASS]/[FAIL] per line with a summary
• Exits non-zero if any expectations are violated (CI-friendly)
• Blank lines and # comments are skipped

Example corpus file

# Test basic grammar rules
✅ This sentence is perfectly grammatical.
❌ This sentance has one or more problems.
❌ `AnA` This is an test.
✅ `SpellCheck` This sentence is correct.

Example output

[PASS] This sentence is perfectly grammatical.
[PASS] This sentance has one or more problems.
[PASS] This is an test.
[PASS] This sentence is correct.

4 total, 4 passed, 0 failed

Closes #2629

Test plan

• cargo test -p harper-cli — all 18 tests pass (9 new corpus tests)
• cargo fmt -- --check — clean
• cargo clippy -- -Dwarnings — clean
• cargo check -p harper-cli and cargo check -p harper-cli --features training — clean
• cargo build -p harper-cli — clean