Apply code review feedback: fix file I/O and parsing bugs

[Copilot]

Addresses code review feedback from PR #2 identifying critical bugs in file parsing and I/O operations.

File parsing fixes (markdown.go)

• Fixed frontmatter detection: s.Scan() must be called before s.Text() to read first line
• Preserve line structure: append "\n" when writing content to maintain original formatting

File I/O and formatting fixes (main.go)

• Use os.Create() instead of os.Open() to actually write output file
• Remove redundant filepath.Join(path) wrapper - path is already absolute from filepath.Walk
• Format SHA256 hash as hex string with fmt.Sprintf("%x", hash) instead of fmt.Sprint() which produces unreadable output like [123 45 ...]
• Move taskName declaration from line 63 to line 123, closer to first usage

Additional

• Added .gitignore to exclude binary artifacts

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