Forward repo metadata to relay fleet placement

[miyaontherelay]

Summary

• forward AgentSpec.repo into normal fleet spawns so relay placement filters by repo
• retain tracked repo metadata on no-session restart spawns
• pin normal multi-repo, restart, durable cold-start, explicit node+repo, and single-attempt placement failure behavior

Validation

• focused: 2 files, 285 tests passed
• full: 38 files, 742 tests passed
• npm run build
• npm pack --dry-run --json (258 files)
• git diff --check

Fixes #84

[gemini-code-assist]

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