Proactive maintenance agent + anomaly detection (0.9.0)

[Vortrix5]

Adds sifty agent run / agent schedule: a background checkup plus anomaly scan that toasts a summary, with an optional auto-fix.

Highlights:

• Anomaly detection (core/anomaly.py): a small local baseline flags a fast free-space drop, junk piling up, or a new startup program.
• Proactive run (core/agent_run.py): notify-only by default. Unattended auto-fix is opt-in ([agent].auto_fix), limited to a hardcoded per-user cache/temp allowlist, never admin or system paths, and never lets the model choose what to delete. Everything goes to the Recycle Bin and stays undoable.
• Home shows anomalies as reviewable rows and, when Ollama is running, a short AI summary of the checkup.

655 tests pass; version bumped to 0.9.0.

Heads up: hold the merge until 0.8.0 is released (Fri 2026-07-17). main needs to stay at 0.8.0 so the scheduled publish still fires, so merge this right after.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 72.43243% with 102 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/sifty/tui/views/home.py	57.84%	36 Missing and 7 partials ⚠️
src/sifty/cli/commands/agent.py	50.84%	23 Missing and 6 partials ⚠️
src/sifty/ai/context.py	37.50%	7 Missing and 3 partials ⚠️
src/sifty/core/anomaly.py	92.52%	5 Missing and 3 partials ⚠️
src/sifty/ai/advisor.py	14.28%	6 Missing ⚠️
src/sifty/core/agent_run.py	91.89%	3 Missing and 3 partials ⚠️

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