Expose native Subagent resources across three harnesses

[SaladDay]

Native child work was not available through the pinned public Subagent resources. This adds all six Subagent GET operations and durable child identity, lifecycle, Turn and Item projection through one Core/Runtime observation contract for Codex, Claude Code and MiniMax Code.

Core owns authorization, scoped pagination and persistence; adapters supply verified native facts. Child Turns remain outside the Core execution queue. Native admission enforces concurrency and the existing workspace/credential boundary. Completion or cancellation does not imply a closed Subagent. Codex cancellation keeps one continuing owner across caller deadlines and permits release confirmation on retry.

Validation:

• Fixed OpenAI Python SDK 3.13.0 and raw HTTP against real Kimi K3 / MiniMax M2.7 execution through independent Core, PostgreSQL and Docker Runtime for all three harnesses: six reads, own histories, scoped pagination and tenant denial.
• Real continuation, child cancellation/workspace-write cessation, Core restart/re-read and native credential/history protection for all three; Codex also verified nesting and native close/reopen.
• Final Codex runtime revalidated public reads, cancellation, same-child continuation and restart recovery. Targeted Codex/dispatch race tests and full make check passed at 8fc5d155a8f15a54447cf76e19bcad98dfa1eb87. Fresh GPT-6 Astra high independent review of the whole diff found no blocking issues.

Scope and limits:

• No public Subagent write API, business Team loop, second scheduler, product compatibility layer or extra execution topology.
• Claude and MiniMax publish child history at settlement; continuous child progress streaming and complete multi-agent compatibility are not claimed.
• Optional native tools differ. Public function/MCP combinations and Codex required ToolEnvironment with Subagents remain unqualified. E2B combinations are not rerun in this batch.
• Native constraints, source pins, bounded MiniMax native patch and evidence references are recorded in contracts/agents-api/subagents.md and adapter documentation. Earlier failed probes remain failed evidence.

Tracked in Feishu: SUBAGENT-PUBLIC-CONTRACT-001. The complete-protocol parent remains open. Stop after this batch is accepted and merged.