docs(readme): rewrite the README for the store

[chhoumann]

Why

The README read like internal notes. It is the first page a stranger sees on GitHub and in the Raycast Store, so it has to sell the extension, show it, and stay true to the code.

What changed

• README.md is rewritten around a hero image, a GIF of the core loop, and one picture per feature.
• demo-vault/ is a small, believable vault with five QuickAdd choices. The pictures and the store screenshots in metadata/ come from it.
• media/ holds the framed screenshots and the GIF. scripts/setup-demo-vault.sh installs the QuickAdd release the demo vault expects.

Scope

• Covers the README, the demo vault, the pictures, and the three store screenshots.
• No code changes. The vault picker is described in text only, because the live picker lists every registered vault by path.
• The Raycast Store badge is a static "coming soon" badge that links to the future store URL.

Verification

• pnpm lint, pnpm test (76 tests), npx tsc --noEmit, and pnpm build pass in the worktree.
• Every screenshot and the GIF come from the dev build driving the demo vault through QuickAdd 2.31.0 and the Obsidian CLI. Each capture also wrote the expected note, checked on disk.
• GitHub's rendered README on this branch resolves every image, checked through the readme API and a request to each raw image URL.

Claims and the code behind them

• One form per choice, text, dropdown, tag picker, date and note pickers: src/interactive-session.tsx, src/form-field.tsx, src/lib/fields.ts.
• Number range and required errors: readField in src/lib/fields.ts.
• [[ and # completion: src/lib/completion.ts, src/completion-pickers.tsx, src/lib/suggest.ts.
• Quick Capture, Capture Selection, Capture Clipboard: src/quick-capture.ts, src/capture-selection.ts, src/capture-clipboard.ts, src/lib/capture.ts.
• Pin as Quicklink and Pin as Quicklink with Argument: ChoiceItem in src/run-choice.tsx, src/lib/quicklink.ts.
• Run in Obsidian: runInObsidian in src/run-choice.tsx, the ui flag in src/lib/obsidianCli.ts.
• "Created", "Added to", "Ran" and Open in Obsidian: doneMessage in src/lib/interactive.ts, doneToast in src/run-choice.tsx.
• Cancel Run and cancelling on leaving the form: CancelAction in src/interactive-session.tsx, abortSession and cancelQuietly in src/lib/interactive.ts.
• Vault detection, opening a closed vault, the same-name refusal: chooseVault, ensureVaultReady, sameNameConflict in src/lib/vaults.ts.
• The three-second Templater message and Open Obsidian: STALL_MS in src/interactive-session.tsx.
• Unknown prompt types: nextEvent in src/lib/interactive.ts, UnsupportedPrompt in src/interactive-session.tsx.
• Recent section of five, the bolt on command choices: ChoiceList and ChoiceItem in src/run-choice.tsx.
• Multi-line text area from type:multiline: fieldSpecFromForm in src/lib/fields.ts, TextControl in src/form-field.tsx.
• CLI path candidates: CLI_CANDIDATES in src/lib/obsidianCli.ts.
• Version requirements: the Version requirement section of ARCHITECTURE.md and the 2.31 message in src/lib/suggest.ts.