fix(mcp): filter schema placeholder echoes and preserve task metadata in task_edit

[souljedi]

Fixes #1006

Summary

When LLM clients call task_edit over MCP to append review comments or update specific fields, some models populate omitted parameters with literal TypeScript schema signatures (e.g. planSet?: string, acceptanceCriteriaSet?[], commentAuthor?: string, commentsAppend?[]: string[]).

Previously, task_edit accepted these strings directly into comment arrays and update fields, cluttering task history with schema type strings. In addition, comment-only updates risked clearing other task metadata if empty arrays or no-op fields were passed.

What changed

• src/utils/task-edit-builder.ts:
  • Added isSchemaPlaceholder to filter echoed TypeScript parameter signatures and type declarations out of string and array update fields.
  • Added isCommentPlaceholder so legitimate short user comments (like status? or title?) are preserved, while signatures like status?: TaskStatus or planSet?: string are discarded.
  • Preserved explicit clear semantics for modifiedFiles: [] while treating whitespace-only arrays as no-ops.
• src/mcp/validation/validators.ts:
  • Handled enum-backed schema placeholder echoes at the MCP boundary so they are treated as omitted without weakening validation of actual enum values.
• src/mcp/utils/schema-generators.ts:
  • Updated task_create schema generation to advertise the configured defaultStatus.
• src/test/task-edit-builder.test.ts & src/test/mcp-tasks.test.ts:
  • Added 9 unit tests for builder placeholder filtering and edge cases.
  • Added full MCP integration tests verifying comment-only updates, metadata preservation, modified-file clearing, and markdown structural validity.

Verification

• bunx tsc --noEmit — clean
• bun run check . — clean (426 files)
• bun test src/test/task-edit-builder.test.ts src/test/mcp-tasks.test.ts — 52 pass / 0 fail (343 assertions)