bridge the host pi config, with the credential by env only

The image ships pi with no provider and no preferences, so a sandboxed run
meant configuring the model twice. ai-sandbox now reads the host
~/.pi/agent/{models,settings}.json and rewrites them instead of copying them:
every provider apiKey becomes "$AI_API_TOKEN" (pi resolves an env reference
itself, so no value is written to disk), '!command' header values are dropped
rather than executed, settings that point into $HOME are dropped because that
path is not mounted, and auth.json / models-store.json / sessions / extensions
are never read.

The rewritten pair is seeded into /home/sandbox/.pi/agent at container start,
owned by uid 1000 and mode 600, and is treated as managed: a host change is
re-seeded, an edit made inside the sandbox is kept and reported, a deletion
takes the host version again.

The token follows the existing environment rules -- one variable, passed by
name through the 0600 env file, announced when auto-passed. A bridged run
whose token variable is unset or empty is refused instead of starting a pi
that cannot reach its model, and the bridge never widens the allowlist: it
warns with the exact --allow to add.

Docs and examples now use a self-hosted gateway (https://aipi.reeseapps.com)
instead of vendor API endpoints, since that is the only thing this tool is
used against. 391 assertions pass (make lint, ./tests/run.sh --e2e).

LLM-Generated: true

config/env.example

@@ -6,6 +6,6 @@
 # Nothing here is passed to the container unless you ask for it with --env or
 # --env-file, so listing a variable is not the same as granting it.
 #
-# ANTHROPIC_API_KEY=sk-ant-...
-# OPENAI_API_KEY=sk-...
-# ANTHROPIC_MODEL=sonnet
+# AI_API_TOKEN=...                       your gateway's token
+# AI_API_TOKEN_FILE=...                  if a tool prefers a file over a variable
+# AI_MODEL=turbo                         model id your gateway serves