feat(gate): add a script kind that runs a policy's own program

[jothimani-rajendran]

What

Adds kind: script to the gate DSL. The declarative kinds (content_regex, forbidden_ref, dependency_allowlist, test_integrity) answer questions a closed table can hold; a check that models flow through a method body (the motivating case is the Java security policy being moved into the catalog) cannot be written that way, and the only door for a script until now was the shell guard, which judges a command and never sees the file being written.

A script gate names a Python file under the policy's implementations/. The runner hands it the same material every declarative kind reads -- staged blobs at commit, the write at tool use and at the turn's end -- as JSON on stdin ({"event", "repo_root", "writes": {path: text}}) and carries back its verdict: exit 0 allows, exit 1 blocks with the script's own stderr/stdout as the reason. Any other exit code, a missing script, or no verdict within 30 seconds refuses rather than allowing what it never judged.

• chock compile rewrites the bare name to the policy-relative path the git hook runs from (build.py).
• chock check refuses a name that is not a bare .py file (schema pattern) and a declared script that is not shipped (checks_gate_shape.py).
• The kind joins WRITE_PATH_KINDS, so it rides the existing pre-tool-use and stop fragments unchanged; a new emitter-stability fixture (stability-script) pins those bytes.
• .chock/bin/gate.py re-vendored from runner.py; docs in spec/gate-dsl.md, docs/concepts.md, CHANGELOG.md.

Definition of done

• chock check → 0 errors, 0 warnings; 5 infos, all pre-existing on main (unchanged by this PR)
• chock check --only matrix passes (32 invariants); no behavior change to existing kinds, matrix unchanged
• chock sync --repo . --check clean
• chock check --only verify clean
• Registry rescanned; no stale entries
• pytest -q green: 1414 passed, 6 skipped (baseline 1394 + 20 new in tests/test_gate_script_kind.py, covering compiled path, commit/tool-use/stop verdicts, crash/missing/timeout refusals, emitter wiring, and validation)
• pytest acceptance/ -c acceptance/pytest.ini --rootdir=acceptance green: 21 passed
• Existing artifacts: none use script, so nothing to migrate; the only touched manifest is the new stability fixture
• Touched manifests: new fixture only, at version 0.0.1; changelog entry added under Unreleased
• ruff check . and ruff format --check . clean

Claims

• No surface is described as enforcing more than it installs. INSTALLED_SURFACES is unchanged: the new kind emits through the same write and stop fragments the existing write-path kind uses, as the stability-script goldens show, and docs/enforcement-surfaces.md needed no change.

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https://claude.ai/code/session_01CzNYfzP8ymU3r4JB9Sz8Ha

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