feat(tui): add 'y' shortcut to copy bean-id (hmans#26)

Adds a new keyboard shortcut y to copy the current bean ID to the clipboard when browsing beans in the TUI.

Changes

- List view: Press y to copy the bean ID under the cursor (ignores multi-select)
- Detail view: Press y to copy the current bean's ID
- Feedback: Shows "Copied beans-xyz to clipboard" in the footer status line
- Help text: Updated help overlay and footer help to include the new shortcut

Implementation

- Uses atotto/clipboard library for cross-platform clipboard support
- Follows existing Bubbletea message-passing patterns
- Status messages clear on next keypress

Details see .beans/beans-govy--add-y-shortcut-to-copy-bean-id-in-tui.md

docs: add OpenCode plugin and integration instructions

- Add beans-prime.ts plugin for OpenCode integration
- Document plugin location options (project-local vs user-global)
- Update README with OpenCode setup instructions

fix(tui): don't quit on Q when create modal is open

Remove viewCreateModal from the list of states where pressing Q
triggers quit, allowing users to type titles containing "q".

fix: remove column headers from TUI list view

- Remove buildColumnHeader() function and its usage
- Keep headers only in `beans list` CLI output

chore: beans!

chore: scrap the stats command

Cleanup

Update README

refactor: use text/template for prompt command

- Replace strings.Builder with text/template for cleaner output generation
- Move dynamic sections (GraphQL schema, types, statuses, priorities) into template
- Rename prompt.md to prompt.tmpl to reflect its template nature

fix!: return error from beans query on GraphQL errors

- executeQuery now returns an error when GraphQL errors occur
- On success, returns just the data portion instead of full response
- CLI now exits with status code 1 on GraphQL errors
- Updated tests to expect new behavior