#1168 Pin the deno runtime for extension checks — bare `deno` is a PATH-hijack surface for the test/lint runtime

Problem

Because the bundled deno path isn't pinned in the primary docs (see sibling issue), agents commonly resolve bare deno to run deno test / deno check / deno doc --lint. That's an execution-path hijack surface:

• A malicious deno earlier on PATH, a ./deno in CWD, or a shell alias/function is executed instead of the vetted bundled runtime.
• These are exactly the commands agents run frequently, often under broad auto-approval — so a hijack runs attacker code with little friction.
• If a swapped binary can produce passing-looking output for deno test / deno check / deno doc --lint, it would defeat the guarantees those commands exist to enforce (hermetic scorer, slow-type lint, deno fmt/deno lint, bundle-time inlining) while appearing to succeed. I haven't verified how much of swamp's scoring re-runs independently server-side vs. trusts local check output — that's the key question to confirm, and it determines severity.

The net effect: the ergonomic gap trains agents into the precise behavior an attacker would exploit.

Proposed solution

1. Everywhere the docs/tooling invoke deno for checks, mandate the absolute ~/.swamp/deno/deno and explicitly warn against bare deno.
2. Add a swamp extension test / swamp extension check wrapper that shells out to the bundled binary itself, so agents never resolve deno from the environment — consistent with the repo's "use swamp, don't bypass it" rule and closing the hijack window entirely.

Alternatives considered

• Put deno on PATH — makes the hijack worse, not better; the fix is an absolute swamp-controlled path.
• Docs-only pinning (sibling issue) reduces exposure but doesn't close it; the wrapper is the durable fix.

Verification status

The PATH-resolution behavior (agents invoking bare deno) is observed. The downstream impact of a hijacked binary on scoring is hypothesized, not tested — flagged here so maintainers can confirm whether local check output is trusted or independently re-verified.

Environment

• swamp skill docs as of 2026-07-15; bundled deno at ~/.swamp/deno/deno.