fix(tracer): gate SDK behind env flag, handle errors gracefully, add shutdown hook - #59

Problem

src/tracer.ts called sdk.start() unconditionally at import time with OTLP_ENDPOINT defaulting to http://localhost:4318. Every run of the service started tracing and attempted to export spans to a local collector , even in development or CI where no collector exists , producing connection errors, overhead, and no way to disable it. There was also no SIGINT handler and no graceful shutdown.

Changes

src/tracer.ts

• isTracingEnabled(env) :pure gate function; returns true only when TRACING_ENABLED=true/1 or OTLP_ENDPOINT is set (non-empty). Off by default.
• initTracing(env) : wraps NodeSDK construction and sdk.start() in a try/catch; any startup error is logged as a warn and the process continues. Has a duplicate-call guard so it's safe to call multiple times.
• Auto-calls initTracing() at import time : no change to existing callers.
• Registers both SIGTERM and SIGINT shutdown hooks (SIGINT was missing before).
• Exports isTracingEnabled and initTracing for direct unit testing.

tests/tracer.test.ts (new, 22 tests)

• isTracingEnabled(): all env combinations , absent, unrecognised, true, 1, endpoint-only, empty endpoint
• initTracing(): disabled path (NodeSDK never called), enabled path (SDK starts), error handling (no throw on start() failure, warning logged)
• withSpan(): success path, ERROR status set on throw, span always ends in finally

package.json

• Added tests/tracer.test.ts to the npm test command so it runs in CI.

.env.example

• Added commented-out TRACING_ENABLED, OTLP_ENDPOINT, SERVICE_NAME.

README.md

• Added OpenTelemetry Tracing section with env var table and Jaeger quick-start example.

Acceptance criteria

• Tracing is off by default ,only starts when TRACING_ENABLED=true/1 or OTLP_ENDPOINT is set
• A missing/unreachable collector does not produce crashes or error spam
• Spans flush cleanly on SIGTERM and SIGINT
• TRACING_ENABLED, OTLP_ENDPOINT, SERVICE_NAME documented in README and .env.example
• All CI checks pass (npm run build ✅, npm run lint 0 errors ✅, npm test 209/209 ✅)

##Closes
closes #43