Add demo-seed for local observability demos

[venzeles]

Adds local demo seed data for the Agent Flight Recorder demo path.

This gives NullWatch a deterministic, repeatable local dataset for observability demos without requiring external services, secrets, or a running agent workload.

Companion UI/proxy PR: nullclaw/nullhub#51

What changed:

• Added nullwatch demo-seed.
• Seeds three representative runs: successful review, tool failure, and handoff/retry.
• Includes spans, evals, costs, token counts, and failure context.
• Keeps seeding idempotent so repeated demo runs do not duplicate data.
• Documents the local demo flow in the README.
• Adds hackathon submission notes.

Validation performed:

• zig build test --summary all
• zig build run -- demo-seed --data-dir "$TMP_DIR"
• zig build run -- summary --data-dir "$TMP_DIR"
• zig build run -- run demo-tool-failure --data-dir "$TMP_DIR"