S3: retrieve — inject top-K skill cards into the worker brief + measure delta (skill-tree #458)

[hadamrd]

Part of #458, depends on S1/S2. Where the speed/accuracy win lands.

Acceptance

• At worker dispatch, the maestro retrieves top-K cards relevant to the ticket (area + semantic) and injects them into the brief.
• Measured: on a repeat-class ticket, injected-cards run shows fewer turns and/or lower tokens than a cold run (record the delta in events) — the win must be real, not assumed (per the 'measure, don't claim' bar).
• Adversarial: a ticket in an area with a known card gets that card in its brief; a ticket in an unknown area degrades gracefully (no card, no error). A deliberately-wrong/stale card is NOT injected.