feat(coding-agent): select subagent thinking levels

[ProDrifterDK]

Summary

• add thinking_level to rlm(...) so parent agents can select a child reasoning level
• keep omission backward-compatible by inheriting and clamping the parent level, while explicit requests fail before admission when unsupported
• document the Python API and cover host forwarding, model capability validation, prompt guidance, and daemon-owned child propagation

Daemon compatibility

This does not change a daemon command, event, or response shape. The kernel host request remains the existing generic rlm.run payload, and the daemon-owned child path already receives the resolved internal thinkingLevel option, so no protocol or schema bump is required.

Validation

• npm run check
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/regressions/4649-subagent-model-selection.test.ts test/system-prompt.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/agent-session-recursion.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/acp-rlm-subagents.test.ts test/suite/regressions/3885-subagent-runtime-host.test.ts
• npx tsx ../../node_modules/vitest/dist/cli.js --run test/daemon-mode.test.ts -t "gives RLM subagents messaging controllers for their own nested children"
• python -m unittest -v test_subagent_registry with the worktree runtime on PYTHONPATH

Note

Add thinking_level kwarg to rlm() subagent spawning

• Adds a thinking_level kwarg to rlm.run that pins the child subagent's thinking level independently of the parent's current level.
• When omitted, the child inherits a clamped version of the parent's level; when provided explicitly, the value is validated against the target model's supported levels.
• Validation is fail-closed: invalid strings, empty values, or model-unsupported levels reject the spawn before admission with a descriptive error.
• Updates the system prompt to document accepted values, inheritance behavior, and fail-closed semantics for the agent's own use.

^{Macroscope summarized f068311.}