Add a native text selection assistant

[mimeding]

Summary

• Captures selected text through macOS Accessibility when the focused field can be verified as safe, without changing the clipboard.
• Adds a compact selection assistant with Summarize, Rewrite, Extract actions, and Ask commands. The selected text is attached to the draft and is never sent automatically.
• Retains the existing assistant-message Copy action and does not modify text in the source application.

Privacy and safety

• Shares one Accessibility text policy with Computer Use so secure-field and readable-role decisions cannot diverge.
• Password, secure, unknown, unavailable, and unverified fields fail closed without posting Cmd+C when the source app is known.
• Missing Accessibility permission has a distinct actionable result instead of being reported as an unverified field.
• Synthetic Copy is reserved for the rare case where macOS cannot identify a source application; its bounded transaction restores the complete clipboard baseline or refuses to publish the capture.
• Native and synthetic captures enforce the same 256 KiB UTF-8 limit.
• Diagnostics include only the source application, character/byte counts, route, and typed outcome. Selected text is never logged.
• The overlay opens at its UI deadline during native capture while preserving the short focus grace required after synthetic Copy.

Validation

• swift test --package-path Packages/OsaurusCore --filter ClipboardContentDiagnosticsTests (35 tests)
• bash scripts/i18n/check.sh
• focused strict SwiftLint
• git diff --check
• unsigned Release CLI and app build with code signing disabled

Manual verification

• Select text in TextEdit, invoke the configured chat hotkey, choose an action, review the generated draft, and send explicitly.
• Verify missing Accessibility permission reports the required recovery action.
• Verify empty selections, secure fields, and unclassified fields show typed outcomes without changing the clipboard.

[mimeding]

Earlier validation completed successfully: focused diagnostics tests and SwiftLint passed, and CI run 29138348141 was green across test-core, test-cli, test-evals, shellcheck, and swiftlint. A current-main repair is now addressing delayed pasteboard updates and rapid repeated hotkey invocations before the PR is presented again for review.