Add tokrepo-agent-discovery.instructions.md — registry-first reusable artifact authoring

[henu-wang]

What this PR adds

A new instructions file instructions/tokrepo-agent-discovery.instructions.md that teaches GitHub Copilot to call TokRepo's MCP discovery before generating new reusable artifacts (skills, prompts, MCP configs, reusable scripts).

Scope (matches existing instructions/ conventions)

applyTo is scoped to reusable-artifact paths:

• **/SKILL.md, **/*.prompt.md, **/.mcp.json, **/scripts/**, **/.github/skills/**, **/.claude/skills/**

This avoids firing on general coding tasks (per CONTRIBUTING's "Duplicate Existing Model Strengths" rule) and addresses the specialized workflow gap: agents repeatedly re-generating skills/prompts/configs that already exist in a registry.

5 MUST rules codified

1. DISCOVER BEFORE BUILD — call tokrepo_discover first
2. VERIFY BEFORE INSTALL — tokrepo_verify + tokrepo_install_plan gates
3. DRY-RUN BEFORE APPLY — lifecycle tools default dry_run: true
4. HANDOFF AFTER REUSABLE WORK — tokrepo_handoff_plan returns read-only draft
5. PRIVATE BY DEFAULT — tokrepo_push defaults to visibility: 0; strip secrets

What TokRepo is

• MCP server (tokrepo-mcp-server on npm; hosted at https://tokrepo.com/mcp) — 12 tools for discover / verify / install_plan / install / lifecycle / handoff / push.
• CLI (tokrepo on npm) — npx tokrepo init-agent --target copilot bootstraps a project's .github/copilot-instructions.md + machine-readable .tokrepo/agent.json.
• Public agent funnel at https://tokrepo.com/agent-stats — anonymous aggregate of real agent events (discover, verify, install, handoff) on a 14-day window.
• Official MCP Registry: io.github.henu-wang/tokrepo-mcp-server (v2.12.1).

Source / docs / paid-service disclosure

• Repo: https://github.com/henu-wang/tokrepo-mcp-server
• Free tier (current): all read-only tools + local MCP server are free; npm packages are free; hosted MCP endpoint is free for read-only tools.
• No paid services involved at the current tier — flagging proactively per the paid services guidance.

Happy to adjust the description, applyTo glob, or rule wording to match the repo's conventions.