refactor: Minor code style improvements (Issue #166)

[raifdmueller]

Summary

Addresses all 5 code style improvements from Issue #166.

Changes

1. ParseWarning.type as Enum

• Added WarningType enum with UNCLOSED_BLOCK, UNCLOSED_TABLE values
• Changed ParseWarning.type from str to WarningType
• Added Enum serialization support to _convert_value()
• Updated CLI and MCP to use .value for JSON output

2. Remove redundant pass in Exception classes

• FileReadError and FileWriteError already have docstrings
• Removed unnecessary pass statements

3. Move datetime imports to top of file

• Removed duplicate from datetime import UTC, datetime inside functions
• Added import at module level in cli.py

4. Extract _compute_hash() helper function

def _compute_hash(content: str) -> str:
    """Compute 8-character MD5 hash for optimistic locking."""
    return hashlib.md5(content.encode("utf-8")).hexdigest()[:8]

• Used in update() and insert() commands

5. Remove stale TODO and update code

• SourceLocation now has end_line field
• Updated API to use elem.source_location.end_line or elem.source_location.line

Files Changed

File	Change
models.py	Added WarningType enum, updated ParseWarning
asciidoc_parser.py	Use WarningType enum
cli.py	Import cleanup, hash helper
mcp_app.py	Use WarningType.value
file_handler.py	Remove redundant pass
api/content.py	Remove stale TODO, use end_line

Test Plan

• All 435 tests pass
• Linting passes
• No behavior changes (JSON output unchanged)

Closes #166

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