Add selector support to command frontmatter

[Copilot]

Commands can now specify selectors in their frontmatter to automatically pull in required rules (database drivers, auth libraries, MCPs, etc.) when invoked. Command selectors combine with task and CLI selectors using OR logic.

Changes

• Added Selectors field to CommandFrontMatter - Commands declare their rule dependencies
• Extract mergeSelectors helper - Eliminates duplication between task/command selector handling
• Updated findCommand - Processes command selectors before rule collection

Example

Command file with selectors:

---
selectors:
  database: postgres
  feature: auth
---

Configure PostgreSQL with authentication.

When a task uses /setup-db, rules tagged with database: postgres or feature: auth are automatically included without requiring every task to manually specify them.

Combined with task selectors:

---
selectors:
  env: production
---

Deploy to production. /setup-db

Rules matching env: production OR database: postgres OR feature: auth are included.

Original prompt

Currently, we ignore command front-matter. This is a problem, a command might need additional tools, MCPs or rules. Any selectors that the command might need should be combined with the task selectors and used to select rules.

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[Copilot]

Pull request overview

This PR adds selector support to command frontmatter, allowing commands to declare their rule dependencies (e.g., database drivers, auth libraries) directly. When a task uses a command with selectors, those selectors are automatically merged with task and CLI selectors using OR logic for rule filtering.

Key Changes

• Commands can now specify selectors in their frontmatter to pull in required rules automatically
• Extracted mergeSelectors helper function to eliminate code duplication between task and command selector handling
• Command selectors are processed during task parsing, before rule collection, ensuring correct filtering behavior

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/codingcontext/markdown/frontmatter.go	Added Selectors field to CommandFrontMatter struct with documentation
pkg/codingcontext/context.go	Extracted mergeSelectors helper and integrated command selector processing in findCommand
pkg/codingcontext/context_test.go	Added comprehensive tests for command selectors and their combination with task selectors
examples/agents/commands/setup-database.md	Added example command demonstrating selector usage
docs/reference/file-formats.md	Documented the new selectors field with examples and usage patterns