docs(harness): document Compose completion watches

[mfpiccolo]

Summary

Document the correct Harness workflow for observing one compose::add operation.

Guidance

• Generate an operation ID before starting the operation.
• Register compose-operation before invoking compose::add.
• Use the exact same operation_id in the trigger and Compose request.
• Set terminal_only: true so progress events do not consume a one-shot completion watch.
• Do not use operation_id: null for per-operation correlation; it is a wildcard subscription.
• Read compose::operation as a race-recovery check.
• Treat terminal: true as the completion marker for both success and failure.

Scope

No active Harness worker code required modification: Harness already consumes __binding correctly. This documentation accompanies the iii change, which fixes Compose-side metadata forwarding.

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[github-actions]

skill-check — worker

0 verified, 70 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.