docs: define v1 requirements
31 requirements across 8 categories:
- Core (6): CRUD, task/thought distinction, completion
- Images (4): attachments, camera capture, view/remove
- Tags (4): add/remove, autocomplete, case-insensitive
- Organization (3): pin/favorite, due dates
- Search (4): full-text, tag filter, date filter, type filter
- Capture (3): quick capture mode
- UX (3): mobile, readable fonts, browser support
- Deployment (4): Docker, env config, volumes
10 requirements deferred to v2
11 features explicitly out of scope
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