feat: replace server instructions with skill resources

[SamMorrowDrums]

Summary

Replaces the server instructions system with MCP skill resources that follow the skill:// URI convention, enabling MCP clients (like pi-mcp-agent) to discover and load domain-specific guidance on demand.

Related: https://github.com/SamMorrowDrums/pi-mcp-agent/issues/2

What changed

Added

• pkg/github/skill_resources.go — 16 skill resources covering all toolsets, registered as skill://github/{name}/SKILL.md static resources
• pkg/github/skill_resources_test.go — tests verifying all tools are covered, content format, URI uniqueness, and registration

Removed

• Server instructions — InstructionsFunc on ToolsetMetadata, generateInstructions(), WithServerInstructions() builder method, and the Instructions field from MCP server options
• pkg/github/toolset_instructions.go — instruction generator functions
• pkg/inventory/instructions.go — the generateInstructions function
• pkg/inventory/instructions_test.go — associated tests

Skills created (16 total)

Skill	Tools covered
context	get_me, get_teams, get_team_members
repos	search_repositories, get_file_contents, list_commits, search_code, etc.
issues	issue_read, search_issues, list_issues, issue_write, etc.
pull-requests	pull_request_read, create_pull_request, merge_pull_request, etc.
code-security	code scanning, secret scanning, dependabot alerts
actions	actions_list, actions_get, actions_run_trigger, get_job_logs
discussions	list_discussions, get_discussion, etc.
projects	projects_list, projects_get, projects_write
notifications	list_notifications, dismiss_notification, etc.
gists	list_gists, get_gist, create_gist, update_gist
users-orgs	search_users, search_orgs
security-advisories	global and repository security advisories
labels	list_label, list_labels, label_write
git	get_repository_tree
stargazers	list_starred_repositories, star/unstar
copilot	assign_copilot_to_issue, request_copilot_review

How it works

1. Skills are registered as static MCP resources with skill://github/{name}/SKILL.md URIs
2. MCP clients discover skills via resources/list (filtering for skill:// URIs)
3. Each skill has YAML frontmatter with name, description, and allowed-tools
4. The markdown body contains usage guidance (previously in server instructions)
5. All tools remain available by default — skills provide context, not gating

Testing

• script/lint ✅
• script/test ✅ (all tests pass)
• New tests verify coverage, content format, and registration