Expands the Agent Skills surface from 2 to 28 bundled skills, adds a
per-repo resource template covering arbitrary GitHub repositories, and
introduces a model-facing discovery tool to bridge the gap left by the
SEP's UI-only completion mechanism.

- Import 25 workflow-oriented skills from github#2374
  as standalone SKILL.md files (skip review-pr and handle-notifications
  due to overlap with our pull-requests / inbox-triage). Migrate URI
  shape to skill://github/<name>/SKILL.md to match github#2374's prefix style.

- Remove the legacy Instructions / InstructionsFunc / WithServerInstructions
  machinery (~300 lines). Skills are now the only guidance surface.

- Add a SEP-aligned per-repo resource template
  skill://{owner}/{repo}/{skill_name}/{+file_path} (from PR github#2129, with
  the non-SEP _manifest endpoint dropped). Gated on a new non-default
  `skills` toolset. Index now advertises both SEP entry types
  (skill-md and mcp-resource-template).

- Add list_repo_skills tool that wraps the existing discoverSkills() and
  returns each discovered skill plus a skill:// URL ready for
  resources/read. Workaround for autonomous-agent discovery on unbounded
  template namespaces — documented as such.

- Add discover-mcp-skills meta-skill (always-on) that teaches the model
  the discover-then-read workflow for both bundled and repo-hosted
  surfaces.

Verified end-to-end against anthropics/skills via stdio JSON-RPC: full
loop from index read through SKILL.md to relative-file resolution works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Adds the SEP-2640-aligned per-repo Agent Skills surface: a single
parameterized MCP resource template that lets any GitHub repo expose
its skills/ directory through the same skill:// URI scheme used for
bundled skills.

Template:
  skill://{owner}/{repo}/{skill_name}/{+file_path}

`{+file_path}` is RFC 6570 reserved expansion, so a multi-segment
relative path inside the skill directory (e.g. references/GUIDE.md)
round-trips through the template as a single value. SKILL.md and any
relative references mentioned inside it both resolve via this one
template handler.

Discovery on the server:
- Recognizes the four agentskills.io directory conventions:
  skills/<name>/SKILL.md, skills/<namespace>/<name>/SKILL.md,
  plugins/<plugin>/skills/<name>/SKILL.md, and root-level
  <name>/SKILL.md (excluding hidden + convention-prefix dirs).
- Skill discovery happens at request time via the Git Trees API.
- skill:// completion is wired through CompletionsHandler so MCP
  hosts can offer interactive autocomplete on owner/repo/skill_name.

Index integration:
- skills.Registry extended with BundledTemplate + AddTemplate +
  EnabledTemplates so skill://index.json now publishes both
  type:"skill-md" entries (the 27 bundled skills) and a single
  type:"mcp-resource-template" entry (the per-repo template).
- IndexEntry.Name is now omitempty since the SEP example shows
  template entries without a name.
- DeclareCapability fires for either type of entry being enabled.

Toolset gating:
- New non-default `skills` toolset gates the per-repo template's
  index entry. With --toolsets=default the per-repo surface is hidden;
  with --toolsets=default,skills (or --toolsets=all) it's published.
- The per-repo template is defined in pkg/github/skills_resource.go;
  registered via AllResources so the inventory wires it up alongside
  repo:// templates.

Adapted from the resource-template work in
github#2129. The non-SEP _manifest endpoint from
that PR is intentionally NOT ported — multi-file skill discovery is
handled by the SEP's per-file URI resolution, not a manifest JSON.

Tests cover: template definition, file handler with SKILL.md, file
handler with multi-segment relative path, missing-arg errors, path
traversal rejection, agentskills.io convention discovery,
URI parsing, completion handler, and the index-entry presence/absence
based on toolset gating.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>