Telemetry: instrument startup path (activation, remote setup, CLI binary)

[EhabY]

Part of the Telemetry Phase A rollout. See the RFC in Linear: #849.

Instruments the startup path from extension activation through workspace readiness.

Implementation PR: #943.

Events

• activation with result, authState, measurement durationMs.
  • Does not include activation phase properties.
• activation.deployment_init for stored deployment initialization, with result, authState, measurement durationMs.
  • This preserves the existing fire-and-forget initialization behavior.
• remote.setup (parent) with result, measurement durationMs.
  • Wrapped in TelemetryService.trace("remote.setup", ...) so all child phases share a traceId.
• Remote setup phases emit as hierarchical child events, not as remote.setup.phase with a phase property:
  • remote.setup.auth_retrieval
  • remote.setup.workspace_lookup
  • remote.setup.workspace_ready
  • remote.setup.agent_ready
  • remote.setup.ssh_config_write
  • Each child event includes result, measurement durationMs, shared traceId, and parentEventId.
• cli.download with result, reason (missing, version_mismatch), measurement durationMs.
  • downloadedBytes is emitted when response body bytes are written, including failed downloads that wrote a partial body.
  • HTTP 304 emits cli.download but omits downloadedBytes because no response body bytes are written.
  • There is no true forced path in the current CLI manager flow.
• cli.verify with result, measurement durationMs.

Sites

• src/extension.ts activation function and stored deployment initialization.
• src/remote/remote.ts setup flow.
• src/core/cliManager.ts download and verify paths.
• src/instrumentation/activation.ts and src/instrumentation/remoteSetup.ts keep startup instrumentation helpers out of the main flows.
• src/telemetry/span.ts and src/telemetry/service.ts provide span-owned property/measurement mutation for values discovered during traced work.

Tests

• Use a TestSink injected via the service container.
• Happy path emits the expected event names.
• Remote phase events share a traceId with the parent and include parentEventId.
• Mid-phase failure emits error on the failing phase and success on prior phases.
• cli.download emits downloadedBytes for HTTP 200 downloads, includes partial bytes on failed downloads after bytes were written, and omits it for HTTP 304.
• cli.verify failure emits the correct error fields.
• Span-owned mutations are included in emitted events, and later caller-owned object mutations are not observed.

Depends on #900, #902.