Rename obs package to observer, update imports, and add CI workflow

[egor-baranov]

Motivation

• Clarify and standardize the observability package name from obs to observer for clearer semantics.
• Update all code and tests to reflect the package rename and ensure callers use the new package path and types.
• Add a GitHub Actions CI workflow to run tests and build release binaries on push and pull_request events.

Description

• Renamed package file obs/obs.go to observer/observer.go and changed the package name from obs to observer while keeping NewLogger and NewMetrics APIs.
• Updated all imports and type references from agentd/obs to agentd/observer across api/, cmd/agentd-*, and tests, including changing Server.Metrics to *observer.Metrics.
• Updated e2e tests in test/e2e/ to construct metrics with observer.NewMetrics().
• Added a CI workflow at .github/workflows/ci.yml that runs go test ./... and builds static binaries for cmd/agentd-api, cmd/agentd-worker, cmd/agentd-node, and cmd/acp-supervisor.

Testing

• Ran go test ./... (as executed by the added CI workflow), and the test suite completed successfully.
• Verified the CI build step compiles the specified binaries (agentd-api, agentd-worker, agentd-node, acp-supervisor) without errors.
• Existing e2e tests were updated and exercised by the test run and passed.