feat(session): add explicit provider registry and target binding
Introduce deterministic deep-provider discovery and immutable per-target routing while retaining disjoint artifact and native capability families.
Expose provider selection consistently through MCP, CLI, environment configuration, session status, prompt completion, typed diagnostics, and generated documentation. Preserve exact provider/profile provenance without request racing or transparent fallback.
Cover overlapping providers, ambiguity, rejection details, cancellation, restoration, package installation, and real Hopper target switching.
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