Adds the kb-config crate per design §6. Provides the frozen Config
schema (§6.4) with serde + toml round-trip, defaults() that exactly
match the reference values (e.g. score_gate=0.30, target_tokens=500,
embedding.dimensions=384, rrf_k=60), and XDG path resolvers that honor
XDG_CONFIG_HOME / XDG_DATA_HOME / XDG_CACHE_HOME / XDG_STATE_HOME.
Layer order in load(): defaults → file → env (KB_<SECTION>_<KEY>);
CLI overrides apply later in kb-cli. Env mapping covers the keys
needed by P0 smoke tests; the rest land as their config sections wire
up.
5 unit tests cover serde round-trip, defaults pinned to design,
KB_RAG_SCORE_GATE / KB_SEARCH_DEFAULT_K env override, and
XDG_CONFIG_HOME handling.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
altair823
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