Rewatch: add OpenTelemetry tracing support

AGENTS.md

@@ -410,6 +410,39 @@ This is useful for iterating on a specific test without running the full suite.
 - **Dependencies**: Inspect module dependency graph in `deps.rs`
 - **File Watching**: Monitor file change events in `watcher.rs`
 
+#### OpenTelemetry Tracing
+
+Rewatch supports OpenTelemetry (OTEL) tracing for build and watch commands. To visualize traces locally, run a Jaeger all-in-one container:
+
+```bash
+docker run -d --name jaeger \
+  -p 4317:4317 -p 4318:4318 -p 16686:16686 \
+  jaegertracing/all-in-one
+```
+
+Then run rewatch with the OTLP endpoint set:
+
+```bash
+OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318 cargo run --manifest-path rewatch/Cargo.toml -- build
+```
+
+Open http://localhost:16686 to view traces in the Jaeger UI.
+
+Note: Use `tracing::debug!` (not `log::debug!`) for events you want to appear in OTEL traces — they use separate logging systems.
+
+##### Honored environment variables
+
+Rewatch follows the OTEL spec for configuration — no rewatch-specific knobs exist.
+
+| Variable | Purpose |
+|---|---|
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | Base endpoint of the collector (e.g. `http://localhost:4318`). `/v1/traces` is appended for the trace exporter. Setting this (or `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT`) is what enables telemetry — if neither is set, tracing is a no-op. |
+| `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` | Full trace endpoint used verbatim. Overrides the general endpoint for traces. |
+| `OTEL_EXPORTER_OTLP_HEADERS` | Extra headers on exporter requests (e.g. `authorization=Bearer xyz`). |
+| `OTEL_SERVICE_NAME` | Service name reported on spans. Defaults to `rewatch`. |
+| `OTEL_RESOURCE_ATTRIBUTES` | Comma-separated `key=value` pairs added as resource attributes (e.g. `deployment.environment=ci,host.name=$HOSTNAME`). |
+| `RUST_LOG` | Controls which span/event levels are captured (e.g. `RUST_LOG=info`, `RUST_LOG=rewatch=debug`). Defaults to `debug` when telemetry is enabled. |
+
 #### Running Rewatch Directly
 
 When running the rewatch binary directly (via `cargo run` or the compiled binary) during development, you need to set environment variables to point to the local compiler and runtime. Otherwise, rewatch will try to use the installed versions:

CHANGELOG.md

@@ -36,6 +36,7 @@
 #### :nail_care: Polish
 
 - Allow builds while watchers are running. https://github.com/rescript-lang/rescript/pull/8349
+- Build system: Add OpenTelemetry tracing support for cli commands.
 
 #### :house: Internal