feat: implement pipeline debug mode with live observability tracing

[nextlevelshit]

Description

Issue #197 was closed but the feature doesn't exist beyond a --debug CLI flag.

What's missing

• No live debug event stream
• No .wave/traces/ trace file collection (mentioned in CLAUDE.md but not implemented)
• No debug tracing system for step transitions, adapter calls, contract validations
• --debug flag exists in run.go but only controls log verbosity

Expected

• --debug should emit structured trace events to .wave/traces/<run-id>.json
• Step-level timing, adapter I/O capture, contract validation details
• Optionally stream to stderr for live observation

[nextlevelshit]

Implemented in efce12e: DebugTracer writes structured NDJSON traces to .wave/traces/.json. Trace points for artifact injection, adapter start/end, contract validation. wave logs --trace reads and displays trace files. 10 tests.

[nextlevelshit]

Reopening — debug mode tracing not implemented.

[nextlevelshit]

Implemented in commit efce12e — structured NDJSON debug tracing with wave logs --trace.

[nextlevelshit]

Auto-closed by CI but debug tracing not implemented.

[nextlevelshit]

Pipeline for #444 failed — keeping open for next iteration.

[nextlevelshit]

Keeping open — pipeline failed, feature not yet implemented.