chore: simplify github readme [ANK-301]

[piazza-bot]

Summary

• Replace the long, outdated GitHub README with a concise repository overview.
• Align the product summary with current site/AnkiWeb positioning.
• Clarify that this public repo contains the GPL Anki add-on while the backend/site live outside this repo.
• Keep support, contributing, changelog, and Intellifiller credit links.

Verification

• Ran ./scripts/build.sh check from /Users/michaelpiazza/conductor/workspaces/smart_notes_all/night_shift_worktrees/ank-301-plugin: formatting, lint, and pyright all passed.
• Ran a review agent loop for the README-only diff; the review agent reported there are no issues.
• Started Anki with the current branch loaded via ./scripts/build.sh sandbox-local. First launch exposed a missing local .venv dependency setup; I installed the pinned requirements.txt into the repo's expected Python 3.11 .venv and reran the sandbox.
• Confirmed Smart Notes loaded successfully in the sandbox: smart-notes.log showed Starting app with info logging enabled, migrations completed, and Local server started on http://127.0.0.1:8766.
• Confirmed the Smart Notes loopback endpoint responded: curl http://127.0.0.1:8766/ping returned {"ok": true}.
• No product UI behavior changed, so no UI screenshot is attached for this docs-only README update.

Agent session metadata

• Working directory: /Users/michaelpiazza/conductor/workspaces/smart_notes_all/night_shift_worktrees/ank-301-plugin
• Session ID: 019e9d01-400c-7552-8264-a71ce3a12be5

[linear]

ANK-301