Add protectedFilePatterns config option

[Tarquinen]

Problem

DCP protects certain tools from pruning (protectedTools), but there's no way to protect file operations based on the file path. Important planning files like plan.md, ARCHITECTURE.md, TODO.md can get incorrectly pruned as "stale" even though they contain critical project context.

Proposed Solution

Add protectedFilePatterns config option:

{
  "protectedFilePatterns": [
    "**/plan.md",
    "**/ARCHITECTURE.md", 
    "**/TODO.md",
    "**/AGENTS.md"
  ]
}

Check tool parameters.filePath against these patterns before allowing pruning in deduplicator.ts and janitor.ts.