feat(agents-api): package private Codex harness with shared metadata access

[SaladDay]

The existing daemon Go RPC can opt into a separately built, exact-pin Codex harness whose private metadata socket uses the same native EnvironmentManager as execution. This provides a deployable consumer of the qualified manager hook while retaining raw stdio and the native model/tool loop.

• Build the independently named Linux amd64 artifact from the pinned upstream commit, canonical shared patch and verified lock/source overlays; retain toolchain and binary provenance.
• Preserve native dotenv/helper bootstrap while freezing private operator selectors first. Bind same-UID metadata access to one remote Environment, reject local fallback, bound frames/requests, stop admission on unresolved native deadlines, and bound runtime shutdown with stdin still open.
• Qualify the final binary through the existing Go caller and real remote executor. Public Files, default selection, Core authority and Claude behavior remain unchanged.

Validation on65c23cc7: full make check467.67s on a clean unchanged tree; nine native tests including isolated dotenv/frozen-binding and open-stdin subprocesses; rustfmt/Clippy; release/source identity; actual binary startup, collision, framing, stalled-peer and EOF faults. Kimi real API workflow75.20s covers four ready owners, nine metadata observations, cancellation with independently observed command exit1.3042s, cold native history/files and released endpoint closure. The private provider-only fixture overlay preserves prompts and assertions. No public HTTP or query change requires separate OpenAPI/sqlc generation.

Retained limitation: the initial MiniMax attempt failed with empty native tool arguments; the loss boundary is unresolved and remains queued. Local exit is not remote descendant retirement. Private metadata is not public Files, filesystem isolation or full protocol compatibility. Fresh independent full-diff review found no actionable issue. All15 final-head CI checks passed; the merged tree matches the verified candidate.