fix(docker): exclude .playwright from rsync staging to avoid error 23

The agent source at /opt/hermes inside the container may contain a
.playwright/ directory with browser dependency files that have restricted
permissions. rsync fails with exit code 23 when attempting to read them,
which kills the container build ("Failed to stage hermes-agent source").

- Add --exclude=.playwright to rsync in docker_init.bash
- Add rm -rf .playwright to cp -a fallback path for symmetry
- Update test_docker_init_excludes_egg_info_during_staging to assert
  both the rsync --exclude and a broad .playwright presence check
- Add a brief note in AGENTS.md Contribution style about mirroring
  directory exclusions in both rsync and cp paths

Fixes #5315

docker_init.bash

@@ -452,6 +452,7 @@ else
       rsync -a \
         --exclude='*.egg-info' --exclude='build' --exclude='dist' \
         --exclude='__pycache__' --exclude='.git' \
+        --exclude='.playwright' \
         "$_agent_src"/ "$_stage_src"/ \
         || error_exit "Failed to stage hermes-agent source to writable build dir"
     else
@@ -460,6 +461,7 @@ else
       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 "$_stage_src"/dist 2>/dev/null || true
+      rm -rf "$_stage_src"/.playwright 2>/dev/null || true
       find "$_stage_src" -type d -name __pycache__ -exec rm -rf {} + 2>/dev/null || true
     fi
     chmod -R u+w "$_stage_src" \