MCP server: expose Frame templates

Motivation

Frame templates give authors a starting skeleton in the web app. An AI client talking to the registry over MCP has no equivalent: it either invents a Frame's structure from scratch or asks the user to go to the web app. Since a large share of Frame authoring will happen in-conversation, templates need to be reachable from the MCP endpoint too.

Depends on Frame templates existing (see the templates issue). Frame creation over MCP is tracked separately in #51.

Proposal

• Expose templates as MCP tools alongside the existing list_frames/get_frame (backend/internal/mcp/resources.go): list the templates available to the caller, and fetch a single template's content.
• Let Frame creation over MCP start from a template, so an assistant can offer "which kind of Frame is this?" and then fill in the skeleton.
• Consider exposing templates as MCP resources as well as tools, matching how Frames are already served both ways for clients that only see one surface.
• Same per-request claims and RBAC path as the read tools; templates visible to a caller follow the caller's org.

Acceptance criteria

• An MCP client can discover and read Frame templates.
• Creating a Frame over MCP can be seeded from a template.
• Behavior is covered by tests mirroring the existing MCP resource/tool tests.