Implement Fusion context_names injection

[dmann000]

Design spec in PR #22 (docs/design/fusion-context-injection.md). Phased:

• Phase 1: Connect-PfbArray -Context + capability-gated injection at Invoke-PfbApiRequest
• Phase 2: Invoke-PfbInContext ambient per-call override
• Phase 3 (optional): Set-/Clear-PfbContext, context_ids, display surfacing

Blocked on: agreeing the design (PR #22) + confirming the capability map records context_names per endpoint.

Owner: TBD after design sign-off.