Design: bundle the first MCP server / custom agent (cross-tool prove-out)

[devantler]

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Part of #38 (Theme 1 — realize "not skills-only").

Problem

AGENTS.md commits the marketplace to bundling any agent resource — "agent skills today; MCP servers and custom agents as they prove out across the supported tools" — and this is standing maintainer direction (devantler-tech/agent-plugins#15 was closed "I do not want this [skills-only scoping]"). Today all 6 plugins are skills-only, so the marketplace's defining cross-tool capability is unrealized.

Proposed direction (design-first, then prove out one)

1. Cross-tool capability matrix. Document how each supported tool consumes a bundled MCP server and a custom agent:
   • Claude Code plugins support .mcp.json (MCP servers), agents/ (custom agents), and commands/ within the plugin source dir.
   • GitHub Copilot CLI / VS Code agent-plugin format — determine its MCP/agent support and manifest shape.
   • Identify the common subset that stays tool-neutral (the marketplace's contract) vs. tool-specific extras.
2. Manifest & CI implications. Decide whether plugin.json / both marketplace.json manifests need new fields (e.g. an mcpServers / agents declaration) and how validate-manifests.sh should validate a non-skill plugin (today it assumes skills/). Keep the two manifests in parity.
3. Prove out the first one. Ship a single, real, useful bundle as the proof — strongest candidate: an MCP server paired with an existing plugin where the skills already assume it, e.g. the Flux MCP alongside gitops-kubernetes (the gitops-cluster-debug skill already drives a live Flux MCP), or the ksail MCP. A custom agent is the alternative first artifact if an MCP bundle proves tool-divergent.

Scope update (2026-06-30): the design split it (as anticipated below), so this issue now tracks
only the design (AC#1 + AC#2), delivered by PR #43 (ADR 0001). The concrete bundle (AC#3) is the
follow-up child #42.

Acceptance criteria

• A short design note (in the issue or a docs//ADR) capturing the cross-tool MCP + custom-agent capability matrix and the tool-neutral subset. — PR docs(adr): decide cross-tool MCP/agent bundling design (ADR 0001) #43 (ADR 0001)
• A decision on manifest/CI changes needed to describe + validate a non-skill plugin resource. — PR docs(adr): decide cross-tool MCP/agent bundling design (ADR 0001) #43 (ADR 0001 §D3)
• One real MCP server or custom agent bundled into a plugin, validated by CI (parity + structure), with README updated. — moved to feat(gitops-kubernetes): bundle the flux-operator-mcp server (first non-skill resource) #42 (flux-operator-mcp into gitops-kubernetes per ADR §D4).

Size

M–L — start with the design note + manifest/CI decision (this issue, closed by PR #43), then the first concrete bundle as the follow-up child #42.