Stabilize git diff sidebar status while switching workspaces

[Dimillian]

Motivation

• The diff sidebar title/subtitle and file summary sometimes flicker when the periodic git status/diff refreshes or when switching workspaces because the hooks cleared state while loading new data.
• The change aims to keep the last-known status/diffs visible per workspace during refreshes to avoid UI flashing.

Description

• Add a per-workspace cache in useGitStatus to store the last successful or error GitStatusState and reuse it when switching to that workspace instead of clearing to an empty state, and persist cached entries on both success and failure.
• Add a per-workspace cache in useGitDiffs to store the last fetched GitFileDiff[] and reuse it on workspace switches to keep the diff list stable while new diffs load.
• Updated files: src/features/git/hooks/useGitStatus.ts and src/features/git/hooks/useGitDiffs.ts with minimal logic to set and read the caches and to avoid clearing state immediately on workspace change.

Testing

• Ran npm run lint, which produced a TypeScript tooling warning from @typescript-eslint about an unsupported TypeScript version but completed the lint step.
• Ran npm run typecheck (tsc --noEmit) and it succeeded with no type errors.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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