Collapse sourceGuid/targetGuid to single-instance-pair shape

[5PK]

Summary

• The CLI no longer supports multiple source/target instance pairs (or multiple standalone GUIDs) in a single run, so this removes the array/comma-parsing scaffolding that was built for that.
• state.sourceGuid/state.targetGuid go from string[] to string; state.apiKeys array replaced with sourceApiKeys/targetApiKeys; deleted dead finalizeAllGuidLoggers.
• push/pull orchestrators collapse to a fixed source+target shape instead of an arbitrary GUID list (removes the stale "future enhancement: handle multiple source/target combinations" TODO in orchestrate-pushers.ts, and the N-way Promise.allSettled fan-out in orchestrate-downloaders.ts is now fixed at ≤2).
• Hard cutover on CLI flags: comma-separated --sourceGuid/--targetGuid now throws a clear error instead of silently using the first value; dropped the plural sourceGuids/targetGuids aliases.
• Reworded the README warning about concurrent CLI instances to reflect that multi-pair-per-run is no longer a concept (only concurrent processes on the same single pair remain a risk).
• ~15 test files rewritten for the new shape; also fixed several latent someString[0] bugs in test seeders (e.g. model-pusher.test.ts) that tsc doesn't catch when narrowing an array field to a string.

Test plan

• tsc --noEmit clean on both tsconfig.json (prod) and tsconfig.test.json (prod + tests)
• Full Jest suite passes: 103 suites / 1841 tests
• Manual grep sweep confirms zero remaining sourceGuid[0]/targetGuid[0]/.length usages outside intentional short-lived local arrays

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