build: move the dev-only source override out of Cargo.toml

Same change as breadbox's fa85fc1. The [patch] block pointing bread-theme and
bread-launcher at ../bread-ecosystem shipped in the committed manifest, where
CI cannot resolve it — the workflow clones this repo alone and runs cargo
--locked with no sibling checkout. Moved to a gitignored .cargo/config.toml.

Hygiene, not a CI fix: bread-launcher does not exist at tag v0.7.4, so the pin
cannot resolve without the override until bread-ecosystem is tagged.

.gitignore

@@ -38,3 +38,6 @@ aster-brief.md
 
 # graphify knowledge-graph output (local tool cache, not for commit)
 graphify-out/
+
+# Local-only source overrides (see .cargo/config.toml).
+.cargo/