Summary

Implements per-agent skill filtering in the <available_skills> tool description, completing the feature originally proposed in #5930.

After #5930 was merged, the maintainer simplified it by removing per-agent filtering from the tool description (permissions were only checked at execution time). This PR adds the infrastructure to pass agent context to tool initialization, enabling the skill tool to filter its description based on agent permissions.

Problem

Without this change, agents see ALL skills in <available_skills>, including ones they can't use (permission = deny). This leads to:

• Wasted tokens listing inaccessible skills
• Agent confusion when it tries to load a denied skill
• Poor UX when certain skills should be hidden from certain agents

Solution

Pass agent context through the tool initialization chain so the skill tool can filter <available_skills> by permissions.

Changes

Infrastructure (tool.ts, registry.ts, prompt.ts)

• Add InitContext interface with optional agent field to Tool namespace
• Update Tool.Info.init signature to accept (ctx?: InitContext)
• Update ToolRegistry.tools() to accept optional agent and pass it to each tool's init()
• Pass agent from SessionPrompt.resolveTools() to ToolRegistry.tools()

Skill Tool (tool/skill.ts)

• Rewrite using explicit Tool.Info<typeof parameters> typing
• Filter accessibleSkills by agent permissions in init(ctx)
• Only skills where Wildcard.all(skill.name, permissions) !== "deny" appear in <available_skills>

Skill Registry (skill/skill.ts)

• Add duplicate skill name detection with warning log

Behavior

With config:

{
  "permission": {
    "skill": {
      "documents-*": "deny",
      "experimental-*": "ask",
      "*": "allow"
    }
  }
}

Agent-specific overrides work as expected:

# In agent frontmatter
permission:
  skill:
    "documents-*": "allow"  # This agent CAN see and use documents-* skills

Testing

• ✅ bun run typecheck passes
• ✅ bun test passes (320 pass, 1 skip)
• ✅ Manually tested with multiple agents and permission patterns
• Backwards compatible: existing tools without init(ctx) usage are unaffected