[codex] Fix start.sh dotenv filtering load

[franksong2702]

Thinking Path

A full-suite quality pass found the start.sh readonly .env regression tests failing on macOS/bash. The existing loader avoided UID/GID readonly crashes, but the source <(grep ...) form produced an empty sourced stream in this environment, so valid keys like HERMES_WEBUI_PORT were silently not loaded.

What Changed

• Filter .env into a temporary file before sourcing it.
• Keep filtering shell-readonly Docker keys: UID, GID, EUID, EGID, and PPID.
• Preserve loading of all non-readonly keys.
• Update the static test wording so it asserts the required contract instead of requiring process substitution specifically.
• Update the changelog.

Why It Matters

start.sh is the documented local launcher. Users can reasonably expect .env overrides such as HERMES_WEBUI_PORT to work even when the same .env also contains Docker UID/GID entries.

Closes #2256.

Verification

• bash -n start.sh
• /Users/xuefusong/hermes-webui/.venv/bin/pytest tests/test_docker_env_readonly_vars.py -q
• Direct bash reproduction of the extracted loader block with:
  • UID=501
  • GID=20
  • HERMES_WEBUI_PORT=8888
  • SOME_KEY=normal-value

The direct check printed PORT=8888, SOME=normal-value, and EXIT_OK.

Risks / Follow-ups

• Low risk: the launcher still sources a filtered copy of the same .env; it only changes the transport from process substitution to a temp file.
• The temp file is removed after successful sourcing. If the sourced .env itself has invalid shell syntax and exits early, a temp file may remain under ${TMPDIR:-/tmp}.

Model Used

OpenAI GPT-5 Codex. AI assistance was used for the quality scan, reproduction, patch, tests, and PR preparation.