feat: add omission placeholder detector for edit and write tools

What

Add a detector that identifies when the model generates lazy placeholder text like (rest of methods ...), // unchanged code ..., or (rest of code ...) in edit and write_file tool calls. Block these at validation time to prevent accidental code deletion.

Why

Models (especially under context pressure) sometimes replace actual code with shorthand placeholders. Without detection, these placeholders get written to disk, silently deleting the code they replace. This is a safety net that catches the problem before it reaches the filesystem.

Ported from Google Gemini CLI's omissionPlaceholderDetector, adapted for Qwen Code's tool validation pattern.

Implementation

• omissionPlaceholderDetector.ts: Normalizes whitespace/comments, matches against known omission prefixes (rest of, unchanged code, etc.) with ellipsis
• edit.ts: Compares placeholders in new_string vs old_string — only blocks NEW placeholders not present in the original
• write-file.ts: Blocks any content containing omission placeholders
• 6 unit tests covering standalone lines, case insensitivity, multi-line, false positive avoidance

Start by reading omissionPlaceholderDetector.ts, then inspect how edit.ts and write-file.ts validate tool calls. Use the six unit tests described in the issue to cover standalone, case-insensitive, multiline, and false-positive cases. Done means new omission placeholders are blocked, while placeholders already present in old_string are not newly rejected.