fix(update): stop repeating the same startup update banner

[694410194]

Summary

• persist last_notified_version when background startup checks surface an available update
• suppress repeat notifications for the same version in both the CLI background notifier and the TUI startup banner until a newer release appears
• add focused cortex-update coverage for notification tracking and persistence

Fixes PlatformNetwork/bounty-challenge#53190
/claim #53190

Verification

• cargo test --manifest-path /tmp/cortex-bounty/Cargo.toml -p cortex-update
• cargo check --manifest-path /tmp/cortex-bounty/Cargo.toml -p cortex-cli -p cortex-tui
• git diff --check

[greptile-apps]

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