Scope prompt-only MCP grants by MCP

[Joncallim]

Summary

Fixes the remaining prompt-only MCP brokerage findings from the landed MCP handoff fixes.

• Scope prompt-only context checks to the same MCP instead of treating any package/agent prompt overlay or subtask as context for every empty grant.
• Keep required empty grants blocked when the only prompt context belongs to another MCP.
• Keep same-MCP prompt-only grants warning-only when the live MCP is unavailable, matching the beta behavior where no live MCP handles are issued and prompt context is passed as planning-only instructions.
• Add regression coverage for the preview and work-package broker paths.

Findings addressed

• Block empty MCP grants without prompt context #171 (comment)
• Allow prompt-only MCP handoff context #170 (comment)
• Block empty MCP grants without prompt context #171 (comment)

Validation

• npx vitest run __tests__/mcp-execution-design.test.ts --pool threads --maxWorkers=1 --no-file-parallelism -> 1 file, 40 tests passed
• npx vitest run __tests__/mcp-execution-design.test.ts __tests__/work-package-handoff-db.test.ts __tests__/repository-evidence.test.ts --pool threads --maxWorkers=1 --no-file-parallelism -> 3 files, 87 tests passed
• git diff --cached --check -> passed

Note: npm run lint -- worker/mcp-execution-design.ts __tests__/mcp-execution-design.test.ts was attempted locally but stayed idle for nearly two minutes, matching the earlier local ESLint hang; it was stopped before opening the PR.