Adds a section-level overlay so config.py reads settings.local.toml
(gitignored) on top of the committed settings.toml. Lets the operator
keep secrets and per-host paths out of git while still committing the
shared template — same pattern other homelab projects use.
One commit (d40f25c): src/second_brain/config.py picks up the overlay
file when present, and tests/test_config_overlay.py covers the merge
semantics. No change to the committed settings.toml format; no secret
introduced into the repo tree.
