feat(runtime): A4.1a deterministic direct admission boundary

[0YHR0]

Scope

A4.1a only: CI/test-only feature gate, immutable TaskRun runtime authority admission, built-in LangGraph v2 registry validation in the caller UoW, migration 0047/ORM constraints, and body-less DIRECT Run admission.

Explicitly out of scope

• No Worker managed-authority execution or ManagedRuntimeExecutionService changes
• No outcome/finalization/reconciliation cutover
• No reviewed/coordinated/generic-subprocess authority cutover
• Gate remains disabled in default profiles and on the test server

Evidence

• ruff check . passed
• A4.1a targeted tests passed: feature/config, TaskApplicationService admission and rollback, TaskRun domain, runtime routes
• Full local pytest had one unrelated intermittent Windows MCP stdio smoke failure; the failing test tests/test_mcp_tools.py::test_official_stdio_client_invokes_the_bundled_read_only_server passed immediately when rerun.
• The initial PostgreSQL assertion failure in this PR was fixed in follow-up commit ab5e3c1 by asserting the expected check violation at statement execution.
• PostgreSQL integration file includes migration/schema/round-trip coverage and is CI-gated by AGENTMESH_RUN_POSTGRES_TESTS=1; local environment skipped those tests. CI must rerun and pass the 42-test integration suite.
• git diff --check passed

Follow-up

A4.1b will separately implement Worker authority execution/finalization and reconciliation after this boundary is reviewed. A4.2 remains open.