feat(server): cross-provider subagent dispatch and model-selection skill

[nohat]

Goal

Allow any T3-hosted agent to delegate work to any configured provider instance and model, independent of the parent agent's provider, model, or project. Pair the shared delegation capability with model-selection guidance grounded in recent T3 usage and available pricing.

Acceptance criteria

• A provider-neutral T3 MCP capability exposes configured provider/model choices and launches delegated work in an explicitly selected project/workspace.
• Delegation records a durable relationship to its parent and exposes status/result through T3's existing agent/thread surfaces.
• Pricing comes from T3's existing cached rate service; unavailable/proprietary rates stay explicitly unpriced. Do not imply API-equivalent pricing is actual subscription spend.
• A portable model-selection skill recommends among models already observed in recent T3 usage, shows sourced pricing, and is available to agents across providers/projects.
• Focused tests cover provider/model/project selection, invalid/unconfigured targets, pricing availability, and completion/status.

Notes

The caller asked for this scope directly. Preserve T3's existing per-project runtime/access decisions; no provider/model/project allowlist is being added.

[nohat]

Implementation is starting in an isolated worktree after fleet admission cleared. Scope: universal MCP tools for model catalog and cross-provider delegated thread launch/status, plus portable model-selection guidance with T3 usage/rate provenance.

[nohat]

Implemented in the isolated worktree at /Users/nohat/code/t3code-provider-agnostic-subagents on feat/provider-agnostic-subagents. Changes are uncommitted and no PR was opened. The shared MCP server exposes list_agent_models, dispatch_subagent, and get_subagent_status to every provider session through the common agents capability. The model catalog includes registered projects, configured provider models, availability, recent 14-day T3 thread counts, and live pricing metadata. The model-selection skill includes the 2026-10-04 usage/rate survey and distinguishes API-list estimates from subscription spend. Same-project child threads reuse the parent worktree; different-project children use the selected project root. Verification: 118 focused tests passed; t3 server typecheck passed; scoped lint, format check, and diff check passed. Leaving this issue open for review and landing.