fix(mcp): separate isolation note from first tool output

[Audgui-Byte]

What does this change?

Add a trailing newline to the one-time MCP isolation note before the first tool result. Add an in-memory MCP client regression test that checks the combined text ends with a newline followed by command output, and that the note does not repeat on the second call.

Why?

Fixes #117. Previously the first command output could appear as isolation.hi when clients concatenate the text blocks.

How to test

• Red/green: the new regression test failed before the change and passed after it.
• vitest run packages/mcp: 42 passed, 1 skipped.
• tsc -p packages/mcp/tsconfig.json --noEmit and tsc -p packages/mcp/tsconfig.build.json: passed.
• node scripts/drift-check.mjs and node scripts/link-check.mjs: passed.
• GitHub CI: all six package jobs across Node 20/22 and macOS/Ubuntu/Windows passed. Two Vercel preview checks require authorization from the upstream team.
• The full monorepo Vitest run was attempted but did not pass in this Windows/pnpm checkout: 1,396 tests passed, 5 failed, and 19 files could not load, primarily in unrelated CLI/SDK/console tests. The MCP package had no failures. A real WSL2/local-provider smoke test was not available here.

Checklist

• npm run build:packages && npm run typecheck && npm test passes locally
• I exercised the changed MCP tool path through a linked in-memory client and server
• New code follows the build contract
• No new dependencies

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