feat: Switchyard OpenCode integration

[drifold]

Stacked on #2026 (base: feature/switchyard-openhands-integration). Adds upstream opencode as a second harness for the Switchyard token-capture integration — same zero-fork principle as the OpenHands path.

What

• Runs opencode headless (opencode run) with an opencode.json provider pointed at Switchyard — no fork, no harness modification.
• Captures the session tree through opencode's own correlation headers: a distinct X-Session-Id per (sub)agent session, x-parent-session-id linking subagents to the root.
• Reconstructs the root session as the main rollout and subagent sessions as token-annotated subagent_trajectories.

Zero-fork architecture

• The opencode.json provider id is deliberately not opencode-prefixed — opencode only emits its native correlation headers for non-opencode providers; those headers are the whole capture contract.
• Gym mints no session ids here: opencode's own ids are discovered post-run from Switchyard's GET /v1/sessions (no harness logs read).
• Title-generator side-calls (same session id, different conversation) are filtered before reconstruction.

Activation and safety

swe_agents:
  responses_api_agents:
    swe_agents:
      agent_framework: opencode
      opencode_source: opencode        # default remains "nv-opencode"
      switchyard_base_url: http://<host>:<port>

condition	behavior
opencode_source absent / nv-opencode	byte-identical to the existing fork path
enabled, capture healthy	root + subagents reconstructed with exact token ids
retrieval/reconstruction failure	rollout kept, mask_sample: true, switchyard_trace_error set

Changes

• responses_api_agents/swe_agents/app.py — opencode_source config, _opencode_switchyard_config (provider JSON), the opencode run command, _list_switchyard_sessions / _reconstruct_switchyard_sessions, title-gen filtering, task_id steering via system-prompt instructions (works around an opencode task-tool crash on model-invented task_ids; upstream fix being filed).
• nemo_gym/switchyard_trace.py — canonicalize tool-call arguments for the history-extension comparison only (opencode re-serializes compact vs. pretty-printed JSON across turns); reconstructed items keep the model's original arguments.
• Tests: 8 new test classes across test_app.py and test_switchyard_trace.py.

Dependencies

Requires the Switchyard side on NVIDIA-NeMo/Switchyard#63: 7b33c2c (native X-Session-Id capture), be0b2c5 (parent session id + GET /v1/sessions).

Testing

• 225 passed across the swe_agents and switchyard_trace suites.
• Cluster E2E (vLLM 0.11.0, Qwen3): root + subagent sessions captured via native headers, both reconstruct with exact token fidelity, 0% masked.

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