chore(release): v0.13.0

[ajianaz]

What

Release v0.13.0 — Runtime embedding config + incremental per-symbol embedding.

Why

Version bump for milestone v0.13.0. Includes:

• BrainConfig runtime embedding selection (auto / hashing / pretrained)
• Incremental per-symbol embedding via embed_fingerprint (schema migration v7)
• Backend enum + resolve_backend() runtime dispatch

Closes #499

Testing

• cargo test --features tree-sitter (903 tests, 0 failures)
• cargo fmt -- --check passes
• cargo clippy --features tree-sitter -- -D warnings passes
• cargo build --release --features tree-sitter passes

Related Issues

Closes #499