feat(capture): add Capture Clipboard

[chhoumann]

Why

Capture Selection needs text selected in the frontmost app. Often the text is already on the clipboard. Capture Clipboard sends it to a capture choice in one step.

What changed

• New no-view command capture-clipboard, with its own Capture Choice preference that uses the same wording as Capture Selection's.
• src/capture-clipboard.ts reads Clipboard.readText(). Empty or whitespace-only text shows a "Clipboard is empty" failure toast. Any other text goes to the shared capture().
• README, CHANGELOG, and ARCHITECTURE name the new command wherever they list the capture commands.

Scope

Plain text only. No HTML or image handling. The text is sent as copied, with no trimming.

Verification

• pnpm lint, pnpm test (73 passed), npx tsc --noEmit, and pnpm build pass.
• pnpm e2e:protocol: all 12 passed, run from the main checkout because the script reads the vault next to itself.
• Live in Raycast: not verified. The deeplink opened Raycast's "Request to run Capture Clipboard" confirmation and then the required Capture Choice form. The Raycast window closed before the preference was typed, so no capture ran (Output/Inbox.md is unchanged). The toast and byte-for-byte checks still need a run.