The module header restated `docs/scheduler/ci.md`'s credential-isolation
paragraph nearly word for word, and re-told the nix-sandbox gotcha that
`docs/process/gotchas.md` owns — both immediately above lines that already
pointed at those docs.
Measured, not eyeballed: 6-gram intersection between this file's comments
and `docs/scheduler/ci.md`, 74 -> 24.
Kept, because neither is visible from the config that implements it and a
reader changing this file needs both:
* the container has its own network namespace, not the host's, so a PR's
nix build cannot reach host-loopback services — a prompt-injection
boundary rather than a preference;
* the forge ADMIN token never enters this container; only a registration
token is handed in.
Each is now one sentence naming the property and its consequence, with the
mechanism behind a `docs/x.md::Section` pointer whose heading was checked
to exist (`Container isolation`, `Container design`).
Also restated in the present: the container is non-ephemeral because the
runner writes its registered credentials on first registration and reuses
them, which is why an ephemeral one would re-register every restart. That
was previously a description of what gitea-actions-runner does, with the
reason left implicit.
Comments only; no behaviour change. Refs #3901.
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