Rename obs package to observer and update references

[egor-baranov]

Motivation

• Rename the observability package from obs to observer to improve clarity and make the package name more descriptive.
• Update all code references to the renamed package to keep imports and types consistent across api, cmd, and tests.

Description

• Renamed package file from obs/obs.go to observer/observer.go and changed the package declaration to package observer.
• Updated all import paths and uses of the package from obs to observer in api/server.go, cmd/agentd-api/main.go, cmd/agentd-worker/main.go, and test files under test/e2e.
• Replaced references to obs.NewLogger, obs.NewMetrics, and the obs.Metrics type with observer.NewLogger, observer.NewMetrics, and observer.Metrics respectively.

Testing

• Ran go test ./... to validate compilation and unit tests, and the test suite completed successfully.
• Verified e2e tests TestServerHealthAndRegistryList and TestSessionEventsSSE under test/e2e executed and passed during the run.