Add local/docs/BUILD-SYSTEM.md as the single authoritative build-system
reference (entry point, CLI flags, pipeline stages, offline/release, caching,
no working-tree stashing, fork/vendored-upstream versioning, self-versioning).
Update AGENTS.md (dirty-gate replaces the removed stash-and-restore; --allow-dirty;
pointer), SCRIPT-BEHAVIOR-MATRIX.md (stash line -> dirty-source gate), README.md
(flags + pointer), and add canonical-doc pointers to BUILD-SYSTEM-INVARIANTS.md
and docs/06-BUILD-SYSTEM-SETUP.md.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
