feat: add Clipboard History plugin

[xcv58]

Summary

• Adds a privacy-first Clipboard History plugin with encrypted local storage and a Keychain-backed encryption key.
• Captures text, rich text, images, PDFs, file references, links, colors, and audio/video clipboard representations.
• Provides keyboard-first search, type filters, rich previews, image OCR search and paste, plain-text paste, quick paste, pinning, deletion, and Finder-compatible file replay.
• Adds password-manager exclusions, configurable app exclusions, Private Copy Now, Ignore Next Copy, and direct plain-text paste shortcuts.
• Supports configurable retention up to 10,000 items and 5 GB, with graceful oldest-unpinned eviction.
• Adds guided onboarding and visually consistent shortcut and retention controls.

Privacy and storage

Clipboard payloads stay on this Mac in encrypted storage. Keychain stores only the encryption key. Sensitive-copy workflows suppress capture before the plugin reads the copied payload, and pinned items are not automatically evicted.

Validation

• Manually exercised onboarding, collection enablement, shortcut assignment, privacy workflows, rich content, image OCR, filtering, search, paste, deletion, and retention settings in MacTools Dev.
• 30 Clipboard localization, plugin behavior, and shared shortcut-layout XCTest cases pass.
• All 86 repository script and PluginKit compatibility tests pass.
• The CI-equivalent changelog and script gates pass. The full parallel XCTest run exposed unrelated timing/network failures; all but AppInstanceCoordinatorTests.testForwardingStopsWhenItsOwningTaskIsCancelled pass when isolated. This branch does not modify that coordinator or its tests.

Screenshots

First-run setup — enable encrypted clipboard collection

Ready to use — required setup is complete; optional shortcuts remain configurable

Sensitive-copy workflow — Private Copy Now configured; Ignore Next Copy remains optional

Closes #306