feat(cli): add --max-file-size to skip large Markdown files (#81)

[luojiyin1987]

Summary

• Add a --max-file-size <size> CLI flag. Markdown files larger than the limit are skipped with a warning: skipped large Markdown file ... message written to stderr, and never reach lint/fix.
• Format: b / kb/k / mb/m / gb/g (case-insensitive). Decimals (1.5mb), whitespace (1 mb), 0, negatives, and invalid strings are rejected with exit 1 + stderr, matching the --threads validation style.
• Filtering happens after loadMdFiles() and before resolveAdaptiveConcurrency(), so --threads auto recomputes concurrency on the remaining files (the two are intentionally independent, per the issue).
• When the flag is omitted, behavior is byte-for-byte unchanged. --fix also skips oversized files. Filtered-to-empty reuses the existing 🎉 No markdown files to lint 🎉 path.

Why this design

• No new stat burst ([cli] getMaxFileSize: chunk stat calls or short-circuit on huge files #80 avoidance): filterFilesByMaxSize reuses runTasksWithLimit with the existing STAT_CONCURRENCY_LIMIT (128), so the per-file stat() pass is bounded exactly like getMaxFileSize. We deliberately did not add another unbounded Promise.all(stat).
• Single responsibility: parse-size.ts parses/validates sizes; configure.ts#getMaxFileSizeOption does CLI validation + exit 1; filter-by-max-size.ts does the bounded scan + warning. The existing getMaxFileSize (real-max-size) is untouched.

Scope / non-goals

• No .lintmdrc persistence, no --ignore-large-files flag, no default threshold (omit = unlimited).
• lintMarkdown() public API unchanged.

Test plan

• parse-size.spec.ts: unit tests for 500kb/1mb/5mb/1gb (+ case), and rejection of 1.5mb/1 mb/0/-1/abc.
• max-file-size.spec.ts:
  • unit: over-limit dropped + stderr warning; all-kept; stat concurrency bounded to STAT_CONCURRENCY_LIMIT; stat failure propagates.
  • CLI (execFileSync): invalid size → exit 1; no flag → old behavior; oversized skipped (warning + not in report); --fix skips; all filtered → 🎉 message; --threads auto + filter on remaining files.
• npm test (88 passed) and npm run lint green.

Closes #81

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