feat(context): add CPG-07 explicit sidecar composition gates

[Wibias]

Summary

• Stack CPG-07 on CPG-06.
• Make live recovery eligibility depend on actual sidecar plans, not coarse parser/config markers.
• Preserve the conservative rollout: active web-search, image, video, or vision sidecars suspend large-result recovery for that turn.
• Preserve the CPG-05/06 review hardening: provider-wire recovery-tool verification and fail-open hidden-usage accounting remain intact.

Scope decision after deep review

The original CP-07 roadmap called this stage "stateless sidecar composition". This PR intentionally implements the safe composition boundary, not a merged recovery+sidecar execution loop. Full recovery composition with web/media/vision remains deferred until CPG-08 has live-provider task-quality, cost-per-success and latency evidence for the simpler stateless recovery path.

This narrowing is deliberate. Until that gate passes, an actual active sidecar remains authoritative and large recovery fails open for that turn.

Replacement note

This replaces PR #22. During stack repair, the CPG-06 base ref was temporarily pointed at the CPG-07 candidate so the repository's existing CPG-06 hardening workflow could be reused. GitHub therefore auto-marked #22 as merged when its head briefly became reachable from its base. The CPG-06 ref was immediately restored and contains no CPG-07 commits. GitHub does not allow reopening a merged PR, so this replacement PR is required for the corrected CPG-07 implementation commit.

TDD

The first commit defines the actual-plan composition-boundary semantics. The implementation commit preserves the earlier wire-safety and hidden-usage fixes while moving recovery preparation behind resolved sidecar planning.

Stack

• Base: agent/context-projection-cp06 / PR feat(context): add CPG-06 stateless recovery-priority loop #21
• Do not merge before its parent.

[github-actions]

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