Catch stale Rust compile drift before push

Add a repository-local pre-push gate that runs the locked Rust workspace build from the repo root, plus concise install and verification docs for maintainers.

Constraint: ROADMAP #694 requires a local installable hook artifact only; branch protection remains external.
Rejected: CI or branch-protection changes | outside the requested local pre-push gate scope.
Confidence: high
Scope-risk: narrow
Directive: Keep .github/hooks/pre-push and contributor docs synchronized if the Rust workspace build command changes.
Tested: bash -n .github/hooks/pre-push; SKIP_CLAW_PRE_PUSH_BUILD=1 .github/hooks/pre-push; git diff --check; python3 .github/scripts/check_doc_source_of_truth.py; cargo fmt --manifest-path rust/Cargo.toml --all -- --check; cargo build --manifest-path rust/Cargo.toml --workspace --locked; .github/hooks/pre-push
Not-tested: shellcheck and markdownlint unavailable in environment.

CONTRIBUTING.md

@@ -33,6 +33,20 @@ cargo build --workspace
 .\target\debug\claw.exe --help
 ```
 
+## Local pre-push build gate
+
+Install the repository-local hook to catch stale compile errors before pushing:
+
+```bash
+git config core.hooksPath .github/hooks
+```
+
+This sets the repo's Git hook directory to `.github/hooks`; if you already use a
+custom `core.hooksPath`, copy or chain `.github/hooks/pre-push` instead. The hook
+runs `cargo build --manifest-path rust/Cargo.toml --workspace --locked` from the
+repository root. If you must bypass it for a non-code/docs-only push, set
+`SKIP_CLAW_PRE_PUSH_BUILD=1`; the hook prints when that escape hatch is used.
+
 ## Checks before opening a pull request
 
 Run the smallest relevant tests for your change, then the broader checks when