test: isolate Pro install from production

[jayzeng]

Summary

• preload a deterministic 503 plugin-service response for the anonymous install CLI test
• stop the Core suite from depending on whether the production Pro preview is deployed
• retain coverage that non-interactive install requires no identity and returns a structured retryable failure

Verification

• bun test test/unit.test.ts
• bun test test/cli.test.ts
• bun run lint
• bun run build
• AGENT_MEMORY_CORE_CHECKOUT=/Users/jay/designland/aimemory/agentmemory bun run validate:local from the private Pro workspace

Data and qmd changes

None. No memory-file format or qmd behavior changes.

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