feat: adopt the Agent Plugins standard and sync skills from prisma/skills

[gregory-boch-prisma]

What

Aligns the plugin with the Agent Plugins open standard (v1.0.0, announced by OpenAI/AWS/Cursor/GitHub/VS Code/Vercel on Aug 6) and syncs skills with prisma/skills at 808913c.

Agent Plugins standard (additive migration)

• plugin.json at the repo root — the portable manifest with the required $schema (https://agent-plugins.org/schemas/1.0.0/plugin.schema.json) and only the standard's closed field set (name, version, description, author, homepage, repository, license, keywords).
• mcp.json at the repo root — the portable MCP declaration for the official Prisma MCP server. The standard requires an explicit transport type; the remote server is declared as streamable-http. (The old .mcp.json's note field is not valid in the standard schema, so it lives only in the legacy file.)
• skills/ is already conformant — each skill dir has a SKILL.md whose frontmatter name matches the directory and uses only standard Agent Skills fields.
• Client-specific directories (.claude-plugin/, .codex-plugin/, .cursor-plugin/, .plugin/) are kept unchanged in structure for Claude Code, Codex, Cursor and the npx plugins installer (which still consumes .plugin/). Per the standard's migration guide the portable root files are the source of truth and legacy manifests can be dropped once all consumers read the standard layout.

Skill sync (prisma/skills 808913c, Aug 13)

• New skill: prisma-mongodb-upgrade (v6 is terminal for MongoDB; stay-on-v6 vs Prisma Next decision guide) with 5 reference files.
• prisma-database-setup and prisma-upgrade-v7 gain the MongoDB routing notes pointing at the new skill.

Housekeeping

• Version bump 0.3.0 → 0.4.0 across all manifests, mongodb keyword, README documents the standard and the new skill.

Heads-up: content staleness (follow-up, belongs in prisma/skills)

While auditing for this sync I found the compute/CLI skills are behind the current product surface — even in prisma/skills:

• prisma-compute is written against the retired @prisma/cli app ... surface (app deploy, app promote, app logs, compute:deploy, create-prisma --deploy, --prod, --no-promote, --create-project). In the unified v8 RC CLI (prisma@next / @prisma/cli@next, 8.0.0-rc.6) the app group is removed: the surface is service / service deployment / service domain / service logs, git connect push-to-deploy, and there is no standalone deploy command. prisma.compute.ts + defineComputeConfig (from @prisma/compute-sdk/config) remain correct, but JSON config is now written with init --config-format json, not --format json.
• No skill covers Prisma Composer (Early Access; composer deploy|destroy|dev|log in the v8 CLI, @prisma/composer 0.10.0).
• No skill covers the v8 RC unified CLI surface (postgres, bucket, contract, migration, agent, envelope prisma.config.ts with definePrismaConfig).

Those need content authoring in prisma/skills first (which is also what prisma agent install pulls from); this PR deliberately only syncs and aligns packaging.

🤖 Generated with Claude Code

[gregory-boch-prisma]

On hold — skill content updates are being handled separately first; this PR (packaging + standard alignment) will be rebased on top once those land.