test(core): cover mcp instruction producer

[kitlangton]

What

Add direct producer-level coverage for McpInstructions.

MCP instructions already reach normal Session execution transitively through SessionContext; this PR does not widen the production Location service surface.

How

Exercise the real McpInstructions layer with controlled MCP catalogs and selected-agent permissions. The suite covers:

• sorted multiline rendering;
• servers with both permitted and denied tools;
• omission when no server tool is reachable;
• global execute denial;
• additive updates;
• complete-list restatement after an instruction change;
• partial removal; and
• complete source removal.

Scope

This is test-only. It does not change production wiring, the durable core/mcp-guidance source key, rendered text, permission semantics, or Session instruction composition.

Testing

• cd packages/core && bun run test: 1,313 passed, 6 skipped
• cd packages/core && bun run test mcp-instructions.test.ts: 3 passed
• cd packages/core && bun typecheck
• bunx prettier --check packages/core/test/mcp-instructions.test.ts
• bunx ast-grep scan -c script/ast-grep/sgconfig.yml packages/core/test/mcp-instructions.test.ts
• bunx oxlint packages/core/test/mcp-instructions.test.ts: 0 warnings, 0 errors
• Repository pre-push typecheck: 32 packages passed