Add structured logging with clog

Implement context-aware structured logging throughout the codebase using
chainguard-dev/clog. This provides better observability with configurable
log levels and structured output.

- Add -log-level flag to control verbosity (debug, info, warn, error)
- Replace all log.Println/Printf calls with structured clog logging
- Thread context through NewProbe, NewProcessor, and NewFileReporter
- Add detailed debug logging for eBPF tracepoint attachment
- Add debug logging for file operations in reporter
- Change "new file" messages to debug level to reduce noise
- Update all tests to pass context.Background()

Default log level is "info" which provides operational visibility without
excessive verbosity. Debug level shows detailed event processing.

Signed-off-by: Jason Hall <jason@chainguard.dev>

plan.md

@@ -6,8 +6,9 @@
 
 Milestone 3 progress:
 - ✅ Prometheus metrics endpoint (`/metrics` on port 9090)
+- ✅ Structured logging with clog
 
-**Next**: Structured logging with clog
+**Next**: Ring buffer overflow handling and metrics
 
 See [Milestone 3](#milestone-3-production-hardening) for remaining tasks.
 
@@ -360,7 +361,7 @@ Volume mounts:
 
 **Deliverables**:
 - [x] Prometheus metrics endpoint
-- [ ] Structured logging with levels (clog)
+- [x] Structured logging with levels (clog)
 - [ ] Ring buffer overflow handling and metrics
 - [ ] Memory-bounded deduplication (LRU or bloom filter for extreme cases)
 - [ ] Health check endpoint