The knowledge repo is cloned with credentials embedded in .git/config;
those credentials survive on disk and the entire LOCAL_DIR (including
.git/) is bind-mounted read-only into every agent container. Any
agent (or prompt-injected PR build) can read the site-admin token via
cat /knowledge/.git/config.
Defense-in-depth layer: in write_nspawn_flags, append
--tmpfs=/knowledge/.git after the --bind-ro=.../knowledge flag whenever
the /knowledge bind mount is present. systemd-nspawn processes the tmpfs
after the bind, overlaying an empty in-memory directory over .git/ inside
the container. Agents see the working-tree documents but not the repo
metadata or stored credentials.
This is independent of the complementary fix in workers/knowledge.rs
(stop embedding credentials in .git/config in the first place). Both
layers together provide defense-in-depth: even if a future credential
accidentally reaches .git/config, it stays off the container floor.
Closes part of issue 2308 (bind-mount isolation layer).
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