fix(status): notify only when root session completes

[TomCC7]

Summary

• Restrict completion notifications to confirmed root-session running-to-idle transitions by adding should_notify_root_session_completion in the status-match path.
• Remove completion notifications from fallback idle transitions (repo unavailable, missing session, and status-fetch error) while preserving needs_inspection updates.
• Add unit coverage for root-only notification gating in src/app/polling.rs.

Verification

• cargo test --lib app::polling
• cargo build

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.35052% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/app/polling.rs	78.35%	21 Missing ⚠️

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