Refactor nexus-core into pure database synchronization infrastructure

Transform nexus-core from a full-featured productivity application into a
focused database synchronization infrastructure library. The project now
provides core sync primitives that apps can build upon, removing all
app-specific features (tasks, habits, pomodoros, focus mode).

## Major Changes

### Architecture
- Remove all productivity features (tasks, habits, pomodoros, time blocking, focus mode)
- Keep only core sync infrastructure: users, devices, oplog, peers
- Simplify CRDT implementation to be generic (apps handle conflict resolution)
- Update from app-specific to infrastructure-only positioning

### Database Schema (001_initial_schema.sql)
- Remove 14 app-specific tables
- Keep 4 core tables: users, devices, oplog, peers
- Simplify oplog from verbose structure to generic JSON-based format
- Schema now: id, device_id, timestamp, table_name, op_type, data

### Code Reductions
- models.rs: 217 → 61 lines (72% reduction, 16 models → 4 models)
- operations.rs: 1007 → 306 lines (70% reduction, 40+ ops → 16 ops)
- logic/mod.rs: 932 → 229 lines (75% reduction)
- lib.rs: 179 → 151 lines (40+ exports → ~25 exports)

### CRDT & Sync
- Remove task-specific conflict resolution
- Make HLC increment work correctly for local and remote operations
- Keep generic oplog and HLC primitives for apps to use
- Apps now implement their own table-specific merge logic

### Tests
- Clean up all 27 obsolete feature tests
- Update migration tests for simplified schema
- Fix CRDT HLC increment test timing issues
- Update oplog structure verification
- All 45 tests passing (18 lib + 5 integration + 9 logic + 10 migration + 3 doc)

### Examples
- Remove p2p_sync.rs (task-specific demonstration)
- Update device_pairing.rs to remove productivity references
- Keep focus on sync infrastructure capabilities

### Documentation
- Update Cargo.toml description and keywords (crdt, libp2p)
- README rewritten to emphasize database sync infrastructure
- Remove productivity app messaging

## Migration Notes
- Deleted migration files: 002, 003, 004 (app-specific schemas)
- Kept migration 001 with simplified core infrastructure
- Apps using nexus-core will need to create their own tables and logic

## Testing
All test suites passing:
- 18/18 library tests ✅
- 5/5 integration tests ✅
- 9/9 logic tests ✅
- 10/10 migration tests ✅
- 3/3 doc tests ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Ru1vly

Cargo.toml

@@ -4,13 +4,13 @@ version = "0.1.0"
 edition = "2024"
 authors = ["FocusSuite Contributors"]
 license = "MIT OR Apache-2.0"
-description = "Cross-platform Rust library for database operations, business logic, and P2P synchronization in the FocusSuite productivity ecosystem"
+description = "Cross-platform database synchronization infrastructure with P2P networking, CRDT conflict resolution, and offline-first architecture"
 repository = "https://github.com/kodfikirsanat/focussuite"
 homepage = "https://github.com/kodfikirsanat/focussuite"
 documentation = "https://docs.rs/nexus-core"
 readme = "README.md"
-keywords = ["database", "sync", "p2p", "sqlite", "productivity"]
-categories = ["database", "network-programming", "authentication"]
+keywords = ["database", "sync", "p2p", "crdt", "libp2p"]
+categories = ["database", "network-programming", "authentication", "data-structures"]
 exclude = [
     "target/",
     ".git/",