Pin the authoring prompts' stable framing

[kikashy]

Goal

Add focused tests for the MCP prompts' load-bearing sentences.

Why

internal/mcp/prompts.go builds the authoring prompts — author_pack, author_graph, test_pack, fix_pack, present_pack, explain_disposition. getPrompt and every build* function is unreferenced by any test.

These are not decorative. They carry claim boundaries a model client acts on: that a pack decides nothing, that a matrix row is a rehearsal rather than a decision, that the conformance claim lives in CONFORMANCE.md and nowhere else. A refactor can drop one of those sentences and every test still passes.

There is precedent for the failure: the author_graph prompt recently told a model to run a tool that cannot see a graph the prompt has not yet had declared, and nothing caught it.

Scope

• Change internal/mcp/prompts_test.go (new file) or an existing MCP test file only.
• For each prompt, assert:
  • it is returned by getPrompt under its advertised name;
  • it is non-empty and mentions the artifact it is about;
  • the claim-boundary sentences it carries are present.
• Prefer asserting a phrase's presence over matching whole paragraphs, so rewording stays cheap and deletion does not.
• Do not change any prompt's wording in this issue. If a prompt looks wrong, open a separate issue.

Acceptance criteria

• Every prompt the server advertises has at least one test.
• At least one claim-boundary phrase per prompt is pinned.
• Deleting a pinned sentence fails a test — check this by trying it locally before opening the pull request.
• No prompt text, tool behaviour, or advertised surface changes.
• env GO111MODULE=on go test ./internal/mcp and go test ./... pass.
• Every commit includes a DCO sign-off created with git commit -s.

Contributor learning

What this runtime does and does not claim, and how a text surface is held in place by tests rather than by care.