feat: honor OTEL_SERVICE_NAME and OTEL_RESOURCE_ATTRIBUTES

[milanagm]

Closes #27.
Fixes LFE-16130

createRuntime() built its NodeTracerProvider without a resource, so a bare provider fell back to defaultResource() only and read neither OTEL_SERVICE_NAME nor OTEL_RESOURCE_ATTRIBUTES. Anything a user set through those standard variables was silently dropped.

The package.json line is not optional

@opentelemetry/resources was already in the tree, but only transitively — under pnpm it is not hoisted, so node_modules/@opentelemetry/ holds just api and sdk-trace-node. pi installs extensions with npm --legacy-peer-deps, which does not install peers. This is the same failure mode that made 0.1.0 unloadable on a fresh install (#17), so the dependency is declared explicitly. Note that no test covers that install path yet — tracked separately in LFE-16130.

Verification

• 64/64 tests green, typecheck clean.
• Two new integration tests. The main one is red without the fix (actual: 'unknown_service:node', expected: 'pi-agent'); the second pins that the default resource is unchanged when the variables are unset, so it passes either way by design.
• runPi now clears OTEL_SERVICE_NAME / OTEL_RESOURCE_ATTRIBUTES alongside the LANGFUSE_* vars, so a developer's own environment cannot leak into a run. Confirmed by running the suite with both exported.
• Verified live against Langfuse Cloud with a paired before/after run

Behavior worth knowing when reviewing

Resource attributes are not purely inert once they reach Langfuse:

• service.version populates the first-class version field on the trace and every observation (visible in the table above).
• deployment.environment / deployment.environment.name set the Langfuse environment when no Langfuse environment is configured. LANGFUSE_TRACING_ENVIRONMENT — and the environment key in langfuse.json — both take precedence, because @langfuse/otel stamps a langfuse.environment span attribute and extractEnvironment prefers span attributes over resource attributes. Invalid values coerce to default server-side rather than being rejected.
• A single malformed entry makes the OTel envDetector discard the entire OTEL_RESOURCE_ATTRIBUTES value, reported only via diag.debug, which nothing here enables.