v0.4.1 — docs/accuracy patch

A small accuracy patch over v0.4.0 — documentation + one error message only, no behaviour change.

Fixes (#16)

• The board's "projection over beads" claim is now consistent. store.py has no separate store of its own — every read/write shells br, and beads-rust owns the .beads/*.db + JSONL. A stale module docstring still called the board "a SQLite data model" (a leftover from before the beads migration); corrected.
• Correct beads-rust reference. The dependency is beads-rust — the br CLI (crate beads_rust), not the stale, write-broken homebrew bd. The README now links the real repo (Dicklesworthstone/beads_rust), recommends cargo install beads_rust, warns off bd, and the "CLI not on PATH" error says the same. (The code already defaulted to br.)

Full diff: v0.4.0...v0.4.1