ci(examples): lint examples in CI and pre-commit

[MarTrepodi]

Summary

Both the pre-commit ruff hook (exclude: ^examples/) and the CI ruff check step skipped examples/, so 28 lint findings had built up there unseen. This clears them and lints examples/ from now on.

• Unused imports and import order (13 findings): applied ruff's safe --fix for F401 and I001. Only unused imports are removed, and each removed name has no remaining reference in its file.
• Dead variable (1 finding): removed fetch_times from examples/Async/streaming_guild_roster_advanced.py. It was created and never read.
• Illustrative results (14 findings): these F841 hits assign a call's result to a named variable to show what the call returns, for example units = await cl.get_game_data(items=DataItems.UNITS). Ruff's unsafe fix would remove that teaching value, so this PR adds "examples/**" = ["F841"] to per-file-ignores with its own comment. It sits apart from the "address incrementally" entries because it's intended policy, not a backlog item. F841 still applies everywhere else.
• Lint coverage: removed the exclude from the pre-commit ruff hook and added examples/ to the CI ruff check paths, matching the format check.

Test plan

• uv run ruff check src/ tests/ examples/ parity/ passes
• uv run ruff format --check src/ tests/ examples/ parity/ passes
• uvx pre-commit run --all-files passes
• All example scripts compile (py_compile)
• A probe file under src/ still triggers F841, so the ignore is scoped to examples/

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