feat: add demo database and video walkthrough script

[bborn]

Summary

• Add cmd/demoseed tool to create a demo database with realistic sample tasks, projects, and memories
• Add docs/VIDEO_WALKTHROUGH.md with complete screencast script including narration, timing, and technical tips
• Add Makefile targets demo-seed and demo for easy demo setup

Details

This PR adds everything needed to create a professional screencast demo of TaskYou:

Demo Database Seed Tool

The cmd/demoseed tool creates a fresh database with:

• 4 sample projects (acme-webapp, mobile-app, infra, personal)
• 12 tasks in various states (6 backlog, 1 queued, 1 processing, 1 blocked, 3 done)
• 12 project memories across different categories
• Sample task logs

Usage

# Create demo database
make demo-seed

# Or run directly with custom path
go run ./cmd/demoseed /path/to/demo.db

# Launch TaskYou with demo database
make demo
# Or: WORKTREE_DB_PATH=~/.local/share/task/demo.db ./bin/task -l

Video Script

The walkthrough document includes:

• Scene-by-scene narration suggestions
• Timing breakdown (~6 minute full demo, ~2 minute short version)
• Key moments to capture
• Keyboard shortcut reference
• Technical recording tips

Test plan

• Demo seed script compiles: go build ./cmd/demoseed
• Demo seed creates database with expected data
• Existing tests pass: go test ./internal/db/...
• Makefile targets work correctly

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