fix(docker): stage agent source to writable build dir before pip install

The Docker smoke gate added in this same PR caught a real production
regression on its very first CI run. v0.51.84 (PR #2470) mounted
hermes-agent-src read-only on the WebUI side and widened the chown
prune to keep the read-only walk happy, but missed that the WebUI's
startup also runs:

    uv pip install "$_agent_src[all]"

against the same now-read-only mount. setuptools' egg_info step writes
hermes_agent.egg-info/ inside the source tree even under PEP 517 build
isolation (this is by design -- PEP 517 isolates the BUILD environment,
not the source tree's metadata directory). On a :ro mount this returns
EROFS, the install fails, error_exit fires, and every multi-container
deploy dies at startup. The smoke gate flagged it on both the
two-container and three-container variants.

The fix
-------
Stage the agent source into a writable build dir under /tmp BEFORE
invoking pip install, then point pip at the staged copy.

  _stage_src="/tmp/hermes-agent-build"
  rm -rf "$_stage_src" && mkdir -p "$_stage_src"
  rsync -a --exclude='*.egg-info' --exclude='build' --exclude='dist' \
        --exclude='__pycache__' --exclude='.git' \
        "$_agent_src"/ "$_stage_src"/
  uv pip install "$_stage_src[all]" ...
  rm -rf "$_stage_src"

The exclusion list matters: when setuptools sees a pre-baked *.egg-info,
build, or dist directory, it takes a timestamp-update code path that
also reads/writes inside that directory -- which itself fails on a :ro
source. Excluding them keeps the build on the fresh-build path
unconditionally.

rsync is in the production image (Dockerfile line 41-44). For users
running custom WebUI images without rsync, the script falls back to
cp -a + post-copy rm -rf of the same artifacts.

Tests
-----
Two new source-level invariants in tests/test_docker_docs_and_readonly.py:

  test_docker_init_stages_agent_source_for_writable_install
    -- asserts _stage_src=... is declared
    -- asserts every `uv pip install ...[all]` line uses _stage_src,
       NOT raw $_agent_src

  test_docker_init_excludes_egg_info_during_staging
    -- asserts the staging path excludes *.egg-info (rsync exclude
       form or cp-fallback's explicit rm -rf both pass)

These would have caught the v0.51.84 regression at the source level
(once written; they're new). The Docker runtime smoke gate is the
durable defence for the broader class of :ro x init-script
interactions, since source-level invariants only catch what they're
written to catch.

Verification
------------
- pytest tests/test_docker_docs_and_readonly.py: 11 passed (9 existing
  + 2 new)
- pytest tests/ -q --timeout=60: 5891 passed, 6 skipped (was 5889;
  delta is exactly the 2 new tests)
- bash -n docker_init.bash: clean

Once this lands, the Docker smoke gate's two/three-container variants
should go green, completing the self-validating loop.

docker_init.bash

@@ -391,7 +391,40 @@ else
     fi
   done
   if [ -n "$_agent_src" ]; then
-    uv pip install "$_agent_src[all]" --trusted-host pypi.org --trusted-host files.pythonhosted.org || error_exit "Failed to install hermes-agent's requirements"
+    # The agent source can be mounted read-only (see docker-compose.two-container.yml
+    # / docker-compose.three-container.yml — the WebUI only reads this volume to
+    # install the agent's Python dependencies and never writes to it). setuptools'
+    # `egg_info` build step, however, touches `hermes_agent.egg-info/` inside the
+    # source tree even under PEP 517 build isolation, which `EROFS`-fails on a
+    # `:ro` mount and (under `set -e`) kills startup of every multi-container
+    # deploy. Stage the source into a writable tmpfs copy so the build can write
+    # its metadata side-by-side without touching the underlying mount.
+    #
+    # The copy excludes any pre-baked `*.egg-info` / `build` / `dist` artifacts
+    # to avoid the timestamp-update path setuptools takes when one is present,
+    # and `--reflink=auto` makes the copy near-free on overlay2/btrfs where
+    # supported. We rebuild on every container start (the agent source can
+    # change across volume re-init); cost is one rsync of ~10MB of Python source.
+    _stage_src="/tmp/hermes-agent-build"
+    rm -rf "$_stage_src"
+    mkdir -p "$_stage_src"
+    if command -v rsync >/dev/null 2>&1; then
+      rsync -a \
+        --exclude='*.egg-info' --exclude='build' --exclude='dist' \
+        --exclude='__pycache__' --exclude='.git' \
+        "$_agent_src"/ "$_stage_src"/ \
+        || error_exit "Failed to stage hermes-agent source to writable build dir"
+    else
+      # Fallback when rsync isn't in the image — straight cp -a, then drop
+      # the build artifacts that would trip setuptools.
+      cp -a "$_agent_src"/. "$_stage_src"/ \
+        || error_exit "Failed to copy hermes-agent source to writable build dir"
+      rm -rf "$_stage_src"/*.egg-info "$_stage_src"/build "$_stage_src"/dist 2>/dev/null || true
+      find "$_stage_src" -type d -name __pycache__ -exec rm -rf {} + 2>/dev/null || true
+    fi
+    uv pip install "$_stage_src[all]" --trusted-host pypi.org --trusted-host files.pythonhosted.org \
+      || error_exit "Failed to install hermes-agent's requirements"
+    rm -rf "$_stage_src"
   else
     echo ""
     echo "!! WARNING: hermes-agent source not found."