Preserve Context Builder pending context through routing

[morluto]

Summary

• promote exact queued Context Builder context when pending-policy routing establishes the run/connection mapping
• preserve it through orderly peer EOF even when the child never calls a context-resolving tool
• restore promoted context exactly once if tentative policy mapping rolls back
• preserve read-file auto-selection handover lineage for replacement connections
• add lifecycle, rollback, and reconnect regression coverage

Root cause

Frozen run context was queued before the child connection existed, but it was only bound during a later context-resolving tool call. A child that initialized/listed tools and exited normally therefore reached teardown with no bound context to detach, and final commit reported missingFinalContext(connection none).

Validation

• ContextBuilderRunLifecycleTests: 12/12 passed
• MCPAgentPolicyAdmissionRaceTests: 32/32 passed
• TabContextRoutingTests: 49/49 passed
• read-file auto-selection suite: 20/20 passed
• strict Swift format/lint: passed
• RepoPrompt coordinated build: passed
• exact-ID ledger verification: passed
• final Codex autoreview: clean

The full pr-ready suite was attempted and stopped on two unrelated tests outside this diff; both passed immediately when rerun in isolation.

Closes #562

[chatgpt-codex-connector]

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