Add a one-command synthetic demo seed for public UI exploration

[cmsjade5000]

Goal

Make it easy for outside contributors to run Vault 966 with synthetic data and see the Library, Vault Health, and match/review workflows without needing private database contents.

The repo already has data/samples/vault966_demo_legacy.csv, docs/demo-data.md, and public-safe screenshots under reports/demo-screenshots/. What is missing is a simple one-command path that seeds a temporary/dev database from synthetic rows and tells contributors which screens to open.

Suggested Scope

• Add or document a one-command demo setup path using only synthetic data.
• Prefer a small script under scripts/ or a Make target that writes to a temporary/dev database, not the live SQLite database.
• Update docs/demo-data.md or docs/samples.md with the exact command and expected pages to visit.
• Keep generated demo outputs out of source control unless they are explicitly public-safe fixtures.

Files or Areas

• data/samples/vault966_demo_legacy.csv
• docs/demo-data.md
• docs/samples.md
• scripts/
• tests/test_demo_screenshots.py or a new focused test for the demo command/docs

Out of Scope

• Copying from the live database
• Using real profile names, real collection titles, logs, .env values, or private exports
• Changing the macOS live service deployment
• Replacing the existing demo screenshots unless the PR explicitly regenerates them from synthetic data

Acceptance Criteria

• A contributor can follow one command or a very short command sequence to create a public-safe demo state.
• The docs explain how to open at least Library and Vault Health using synthetic data.
• The command does not touch ~/Library/Application Support/Vault966/data/vault.db or the repository vault.db symlink.
• Tests or documentation checks cover the new demo path enough to prevent drift.

Verification

• Run the new demo command from a clean-ish checkout.
• Confirm the generated/demo database is temporary or clearly separate from live data.
• Run the focused test added by the PR, plus pytest tests/test_demo_screenshots.py if the screenshot docs are touched.

Contributor Notes

This is a high-impact onboarding issue for outside contributors. First-time contributors are welcome: fork the repository, keep the script/docs scoped to synthetic data, and open a pull request that links this issue.