#179 refactor(t3): remove the t3-v1-steer target and T3 V1 store support

Current T3 Code runs only its newer engine and keeps its data in statev2.sqlite. Swarmail still carried code for the old engine: a fallback to the old state.sqlite database, old-format SQL queries, and a t3-v1-steer wake target that posted to a route current T3 no longer has. None of it can work against a current T3 install, so this removes it.

What changes:

• Swarmail reads only statev2.sqlite. While T3 is still creating that file on its first start and it has no thread tables yet, Swarmail sees no threads, and pr-sessions says "local T3 store has no thread tables yet".
• The t3-v1-steer wake target is gone. A configuration that still names it fails when it loads, with: "target type t3-v1-steer was removed: current T3 Code runs only its V2 engine, which has no V1 route; use t3-v2-queue".
• Docs (wake-bridges, usage, pr-sessions) no longer describe the old engine. The changelog notes the removal.
• Tests: three tests that only covered the old engine are deleted, the automatic T3 wake tests now use new-engine fixtures, and a new test checks that a t3-v1-steer configuration fails at load and names its replacement.

Checks: bun run check passes, and bun test gives 1295 pass, 7 skip, 0 fail.

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Summary by cubic

Removes the t3-v1-steer wake target and all T3 Code V1 store support. Current T3 Code runs only its V2 engine, so the old bridge failed its thread check and could never deliver.

• Swarmail now reads only statev2.sqlite; a store T3 is still creating has no thread tables yet, so pr-sessions reports that instead of "T3 V1 is unsupported".
• Configurations naming t3-v1-steer fail at load with a message directing them to t3-v2-queue. A bridge switching targets must stop the old bridge and discard its saved state, which binds to the old destination, before starting it again.
• T3 thread discovery now requires exactly one thread whose provider and native session ID match. Docs, changelog, and tests cover the removal.

Summary by CodeRabbit

• Breaking Changes
  • The t3-v1-steer wake target is no longer supported. Configurations using it fail at startup; switch to t3-v2-queue.
  • T3 thread discovery and session tools now rely on the V2 database. If it is missing or not ready, no threads are found; pr-sessions exits with an error.
• Documentation
  • Updated setup and usage guidance to reflect supported wake targets and T3 thread discovery through provider and native IDs.