fix: reduce background refresh energy use for V2.2.2 - #83

Summary

Reduces idle Extension Host work before V2.2.2. Unfocused VS Code windows now
suspend polling plus Claude/credential file watchers and perform one catch-up
refresh when focus returns. Repeated quota authentication failures also use an
exponential backoff capped at one hour, reset immediately when credentials
change.

Linked issues

None (follow-up to the local energy investigation for V2.2.2).

Type of change

• Bug fix (non-breaking) → fix (patch)

How was this tested?

• TypeScript compilation completed successfully.
• Full suite: 177 passed, 0 failed, 1 filesystem-capability skip.
• Added transition tests for background suspend, foreground resume, timer,
  Claude watcher, credentials watcher, and quota backoff behaviour.

Checklist

• npm run compile is clean
• No user-facing strings were added
• No README behaviour or setting contract changed
• CHANGELOG.md updated (user-visible changes)
• No new settings were added
• No new runtime dependencies

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