refactor: replace fetch_instructions with skill tool and built-in skills

[hannesrudolph]

Summary

Replaces the fetch_instructions tool with a new skill tool and built-in skills system, following the same pattern used by built-in slash commands. This simplifies the codebase by consolidating the "just-in-time instruction delivery" mechanism into the existing skills system.

Changes

• Add new skill native tool (src/core/tools/SkillTool.ts, src/core/prompts/tools/native-tools/skill.ts)
• Add built-in skills (create-mcp-server, create-mode) in src/services/skills/built-in-skills.ts
• Update SkillsManager to merge built-in skills with user skills (precedence: project > global > built-in)
• Remove FetchInstructionsTool and related infrastructure:
  • src/core/tools/FetchInstructionsTool.ts
  • src/core/prompts/tools/native-tools/fetch_instructions.ts
  • src/core/prompts/instructions/ directory
• Remove enableMcpServerCreation setting and UI checkbox (no longer needed since skills are always available)
• Remove auto-approval logic for fetchInstructions
• Update ChatRow.tsx to display skill loading with name, source badge, and description
• Update tests to account for changes

How It Works Now

1. The agent sees create-mcp-server and create-mode in the AVAILABLE SKILLS system prompt section
2. When the task requires creating an MCP server or custom mode, the agent calls the skill tool with the skill name
3. The SkillTool resolves the skill from built-in skills or user-defined skills (via SkillsManager)
4. The skill instructions are returned to the agent to follow

Decisions Made

1. Generic instructions for paths: Skills are static content, so dynamic path resolution (like getMcpServersPath()) was replaced with generic instructions
2. Skills available in all modes: Granularity can be added later if needed
3. Override precedence: Project skills > Global skills > Built-in skills
4. New skill tool: Instead of removing tools entirely, the fetch_instructions tool was replaced with a more flexible skill tool that works with the unified skills system

[roomote]

Oroocle   See task on Roo Cloud

Re-review: built-in skill loading and webview UI typing issues look resolved. Remaining item is CLI tool rendering support for the skill tool.

• Built-in skills no longer require file-based <location> that would break read_file
• UI/types recognize and render the skill tool approval payload
• CLI tool renderer should recognize/label skill (still references fetchInstructions) (apps/cli/src/ui/components/tools/types.ts:18, apps/cli/src/ui/components/tools/utils.ts:48)

Previous reviews

• d83a9c5: Review #1
• a0d5728: Review #2
• b6f7fe0: Review #3
• 99b424c: Review #4
• d876abc: Review #5

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[roomote]

SkillTool sends the approval payload as { "tool": "skill", ... }, but ClineSayTool in @roo-code/types does not include a "skill" variant and the webview ChatRow renderer has no case for it. In practice this can render as an unknown tool request (or fail type-safely elsewhere) when the UI tries to parse/display the approval.

_{Fix it with Roo Code or mention @roomote and request a fix.}

[roomote]

skill was added to packages/types/src/vscode-extension-host.ts:780, but the CLI tool renderer still lists fetchInstructions and does not recognize skill (see apps/cli/src/ui/components/tools/types.ts:18 and apps/cli/src/ui/components/tools/utils.ts:48). If the CLI is used, skill tool approvals/results can render as unknown/miscategorized.

_{Fix it with Roo Code or mention @roomote and request a fix.}