Explore and implement clipboard context support

[FuJacob]

Problem

The clipboard often contains useful context for the thing a user is writing, such as copied notes, a link, a paragraph, or a draft. Tabby currently ignores clipboard context, so completions can miss obvious nearby information the user intended to use.

Goal

Add an opt-in clipboard context pipeline that reads clipboard contents locally, decides whether it is relevant, and injects a compact context summary into supported prompt modes.

Proposed Scope

• Explore how often clipboard contents are useful versus stale or sensitive.
• Add a global setting for clipboard context.
• Respect per-app disabled rules and secure-field blocking.
• Read clipboard content locally and avoid storing it by default.
• Summarize or truncate clipboard content before prompt injection.
• Add clear UI copy that clipboard contents are processed locally and can be disabled.
• Decide whether clipboard context should apply only to Use My Instructions mode or all engines.

Acceptance Criteria

• Clipboard context is off by default or clearly opt-in.
• When enabled, Tabby can include a bounded clipboard summary in generation requests.
• Clipboard context is never used for secure fields or disabled apps.
• Large clipboard contents are truncated or summarized safely.
• The feature can be disabled from Settings.
• The prompt preview or diagnostics make it clear when clipboard context was included.

Open Questions

• Should clipboard context require an explicit per-suggestion action instead of running automatically?
• How should Tabby detect stale clipboard contents?
• Should URLs, images, files, and rich text be handled differently from plain text?
• Should clipboard context be stored for personalization or treated as ephemeral only?

[FuJacob]

Done #105