chore(lint): enable high-signal correctness checks

[Zoupers]

Summary

• enable Ruff B005, B023, B904, B905, and RUF006 in the default lint configuration
• retain fire-and-forget asyncio tasks and cancel the Explore Lab startup task during shutdown
• preserve exception causes, make zip() length assumptions explicit, and remove JSON code fences without str.strip() character-set semantics
• add a regression test for fenced summary content and prevent loop-closure regressions like the duplicate restored alarm scheduling bug fixed in fix(alarm): prevent duplicate restored triggers #117

Validation

• uv run --frozen ruff check src tests scripts apps/explore-lab/backend — passed
• uv run --frozen mypy src — passed (139 source files)
• affected root unit tests — 490 passed
• Explore Lab branch runner tests — 20 passed
• git diff --check — passed

Risks and compatibility

• no public API or configuration compatibility changes
• background task references are released on completion; Explore Lab now waits for startup-task cancellation during shutdown
• externally supplied Chroma result arrays retain truncating zip(..., strict=False) behavior, while internal parallel arrays use strict=True

Checks not run

• the complete repository pytest suite was not run locally; the directly affected root and Explore Lab suites were run

Related: #117