Avoid duplicate Tasks startup reconciliation

[bb-slop-cop]

Human comments

What was wrong

Tasks lifecycle registration awaited the same tracked-thread reconciliation twice. Each pass performs one SDK lookup for every tracked row that remains non-terminal, so startup repeated the full lookup pass without any intervening work. The direct reproduction and root-cause report is at https://get-bb.github.io/reports/issues/3209.html.

What changed

Removed the duplicate startup reconciliation call and added a focused lifecycle regression test that requires one lookup for one non-terminal tracked thread. Updated existing lifecycle expectations and periodic-sweep setup to reflect the intended single startup pass. This changes 20 text lines across 2 files and does not change any wire contract, CLI surface, dependency, generated file, migration, or stored data.

How you verified

• The focused regression test failed on unchanged origin/main: one tracked thread recorded two threads.get calls instead of one.
• The same failure reproduced in a second clean checkout at 06aeaa994942ae7527dc49d2268c1f801e8542a0 after a frozen install and full build.
• The focused regression test passes after the fix.
• pnpm exec turbo run test --filter=bb-plugin-tasks — 35 files and 365 tests passed.
• pnpm exec turbo run typecheck --filter=bb-plugin-tasks — passed.
• pnpm exec turbo run build --filter=bb-plugin-tasks — passed.
• git diff --check — passed.
• The required pre-push EAP codename scan was clean for tracked content, changed lines, and the commit message.

Fixes #3209

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