Support T3-owned delegation in pstack provider dispatch #253

Current scope

Support T3 Code as a third parent harness, next to Claude Code and Codex (Eric, 2026-11-06). Start from PR #254, ported to a branch in this repository with fikaventures as co-author.

Acceptance:

1. provider-dispatch.md has a T3 parent route: pstack picks each role's model and effort. A lane whose provider T3 offers and has enabled goes through T3's delegate_task; any other lane goes through the existing external runner (Eric, 2026-11-06: "any lane for which there is a delegate provider available and configured should go through that delegate provider, otherwise it should go through the external runners"). A named dropout only when neither route can run it. Nothing is silently substituted.
2. /setup-pstack works from a T3 session.
3. AGENTS.md, UPSTREAM.md and .agents/triage-backlog.md name three harnesses, and drop the rule that routes a new tool only as an external provider.
4. verify-open-pstack can run a live lane from a real T3 session, and the live gate for this PR covers T3, Claude Code and Codex.
5. Gates pass: bun run test, bun run typecheck, bun run static, claude plugin validate plugins/pstack.

Execution brief

Unit of the 2026-11-06 batch. Design: ~/projects/docs/projects/open-pstack/records/2026-11-06-batch-262/design-253.md. Opens after #268 and its fix-forward #270 merge, because it edits AGENTS.md. This session runs in T3 Code, so T3 live tests run here.

Pstack's provider dispatch currently assumes a standalone Claude Code or Codex parent. In T3 Code, host orchestration instructions require T3 delegation for cross-provider models, requested settings, worktrees and background tasks. The conflicting routes cause sessions to bypass the CLI runner without a shared evidence contract.

Add a T3 route at the existing provider-dispatch boundary. Pstack retains role/model/effort selection and review methods. The parent invokes T3's MCP tools; T3 owns worktree creation, lifecycle and cancellation. Standalone routes and the CLI runner stay intact.

Deliver a small JSON request compiler and result normalizer, host-aware dispatch guidance, and repeatable boundary verification. Separate T3 launch evidence from CLI receipts and provider-served-model proof. Refuse unsupported strict enforcement requirements instead of silently weakening them. Do not add an HTTP bridge, CLI fallback, recursive orchestration, global configuration changes, implicit deadlines or model substitution.

Verification will exercise the helper as a process, then pass its request to the real T3 delegation tool and normalize the actual terminal response. Preserve compiled requests, responses, artifacts and repeatable assertions. Existing standalone behavior receives focused regression checks.