[enhancement] Add custom tool override capability for domain-specific operations

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Background

The Copilot SDK v0.1.30 (released March 3, 2026) adds support for overriding built-in tools like grep, edit_file, or read_file, implemented by Steve Sanderson in commit f843c80.

This feature allows applications to customize how Copilot agents interact with files and code, enabling domain-specific optimizations and constraints.

Proposal

Leverage tool overriding to implement Planeteer-specific file operation constraints and workspace scoping:

1. Override edit_file tool to:

   • Enforce that agents only edit files within the current working directory
   • Prevent accidental modification of system files or directories outside the project
   • Add pre/post-edit validation hooks (e.g., syntax checking, formatting)
   • Log all file modifications for audit trail in the Execute screen

2. Override read_file tool to:

   • Implement intelligent file size limits (warn/truncate for files >100KB)
   • Cache frequently-read files to reduce redundant operations
   • Track which files each task has accessed for dependency inference

3. Override grep tool to:

   • Scope searches to project-relevant directories (exclude node_modules, .git, etc.)
   • Add search result caching for repeated queries
   • Provide more context lines by default for better agent understanding

4. Configuration in Settings - Add .planeteer/config.json to allow users to enable/disable overrides or customize behavior

Benefit

• Safety: Prevent agents from accidentally modifying files outside the project workspace
• Performance: Caching and scoping reduce redundant operations and improve execution speed
• Observability: Better logging and tracking of tool usage during execution
• Customization: Users can tailor tool behavior to their project requirements

Acceptance Criteria

• src/services/copilot.ts registers custom tool overrides with overridesBuiltInTool: true flag
• edit_file override validates paths are within process.cwd() before executing
• read_file override implements size limits and caching logic
• grep override excludes common ignore patterns (node_modules, .git, etc.)
• Settings file .planeteer/config.json allows enabling/disabling overrides
• Execute screen displays tool usage statistics (reads, edits, greps per task)
• Tests in src/services/copilot.test.ts verify override behavior
• Documentation in README explains tool overrides and configuration options

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