AGE-1407: Migrate Close to the Agent Plugins standard

[nickpersico]

The Close plugin currently uses Cursor-specific manifest, MCP, and command files. This migrates the existing repository to closeio/agent-plugin with a portable Agent Plugins 1.0.0 package that can serve as the base for compatible clients and marketplaces.

• Replace .cursor-plugin/plugin.json and .cursor-mcp.json with the standard root plugin.json and mcp.json, using Close's hosted streamable-http MCP server and client-managed OAuth.
• Convert account briefs, pipeline reviews, and follow-up plans from Cursor commands to portable Agent Skills alongside the existing close-crm skill. Replace $ARGUMENTS with the user's request context.
• Update installation, migration, contribution, and security documentation. Retain brand assets for marketplace listings and bump the plugin to 0.2.0.
• Validate against vendored official schemas and parsed skill frontmatter; add regression tests and GitHub Actions. Dependencies are development-only, and vendored schemas retain their Apache-2.0 license.

Validation: npm ci, npm run validate, all 15 npm test cases, the skill-creator validator for all four skills, and git diff --check pass. An unauthenticated MCP initialize request returns HTTP 401 with the expected protected-resource discovery URL; that resource advertises Close's authorization server and scopes.

Target-client installation, skill discovery, and a full OAuth sign-in still need a smoke test before marketplace submission. The Cursor marketplace entry is handled separately; this PR does not publish or update marketplace listings.

Implements AGE-1407.

[linear-code]

AGE-1407