feat(gate): P17 keeps the engine tool surface out of skill content

The first-party agent-tool layer is Experimental and its surface moves between
builds. A recipe that names a toolset does not fail loudly on the next build --
the tool is simply absent, the search finds nothing, and "nothing found" reads
as "no problem here". That is the exact confusion this bundle documents, so the
tokens are barred rather than discouraged.

Grounded in measurement against a live editor, not in reading:
- the aggregator plugin lists 21 dependencies; the server reported 53
  registered toolsets from 22 plugins, one dependency contributing none;
- the visible tool count flips between 3 meta-tools and every tool registered
  natively, on one project setting.

- gate.py: ENGINE_TOOL_TOKENS and check_engine_tool_surface, registered in
  CHECKS; rule floor raised to 17.
- test_gate.py: one poison naming a toolset, a meta-tool and a host:port.
- ADR-0004 records the decision, its confirmation criteria and what is
  deliberately deferred.
- README and CONTRIBUTING state the rule where a contributor meets it.

Verified: gate.py 0 violations over 17 rules; test_gate.py 17/17 redden on
their fixtures, baseline clean, surface coverage intact. The token list matches
nothing under skills/ today, checked before the rule landed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -63,8 +63,10 @@ claude plugin validate . --strict
 
 The gate is not advisory. It enforces, among other things: no absolute paths, no source
 citations, no donor project name outside a Provenance section, no Cyrillic, no vendor or
-harness name anywhere under `skills/`, and the six required fields on every failure-mode
-entry.
+harness name anywhere under `skills/`, no engine toolset, meta-tool or endpoint name
+either (P17, see
+[ADR-0004](docs/architecture/decisions/0004-engine-tool-surface-out-of-skill-content.md)),
+and the six required fields on every failure-mode entry.
 
 If you add a rule to the gate, **add a poisoned fixture that makes it fire.** The
 self-test fails if any declared rule has no fixture. This is deliberate: the validator