feat: support running skills as isolated sub-agents via context: fork

[dgageot]

Summary

Add context: fork SKILL.md frontmatter support, matching Claude Code's syntax. When set, the agent spawns an isolated child session for the skill instead of reading its instructions inline.

Changes

• Extract shared sub-session primitives from the duplicated handleTaskTransfer/RunAgent logic into reusable building blocks: SubSessionConfig, newSubSession, runSubSessionForwarding, runSubSessionCollecting
• Add Context field and IsFork() to skills.Skill for parsing context: fork from SKILL.md frontmatter
• Add run_skill tool to SkillsToolset, exposed only when fork skills exist
• Add handleRunSkill runtime handler that builds a child session with the skill content as system prompt and the task as user message
• Extend SubSessionConfig with SystemMessage and ImplicitUserMessage overrides for custom sub-session prompts
• Update Instructions() to tag fork skills with <mode>sub-agent</mode> and explain run_skill usage
• Document the feature in docs/features/skills/index.md with a frontmatter fields reference table and usage guide

How to use

Add context: fork to a SKILL.md frontmatter:

---
name: bump-go-dependencies
description: Update Go module dependencies one by one
context: fork
---

The agent will use run_skill instead of read_skill, spawning an isolated sub-agent session with its own conversation history.

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

Summary

This PR introduces context: fork support for running skills as isolated sub-agents, matching Claude Code's syntax. The implementation refactors the existing agent delegation logic to extract reusable sub-session primitives and adds a new run_skill tool for fork-mode skills.

Code Quality: The refactoring is well-structured, extracting duplicated session-construction logic into SubSessionConfig, newSubSession, runSubSessionForwarding, and runSubSessionCollecting. This improves maintainability and sets up a clean foundation for the new skill runner feature.

Testing: Comprehensive test coverage added for the new primitives (agent_delegation_test.go) and the skills toolset changes (skills_test.go). Tests cover edge cases like custom system messages, implicit user messages, agent pinning, and fork skill detection.

Documentation: The feature is well-documented in docs/features/skills/index.md with a clear frontmatter reference table and usage guide explaining when to use context: fork.

Findings

No bugs found in the changed code. The refactoring correctly preserves the existing behavior while adding the new fork skill capability.

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Note: This is an automated review. The assessment label (🟢 APPROVE) is informational only — this bot always posts reviews as COMMENT and never grants merge authority.