feat: add custom environment keys in dashboard

[CRMbyRSM]

Summary

• Adds an “Add custom key” form to the dashboard Keys page for arbitrary environment variables.
• Shows custom keys already present in .env in a dedicated Custom keys section.
• Preserves existing curated provider/tool keys while marking unknown .env entries as custom metadata.
• Adds duplicate-key warning and switches the action to Replace when a key already exists.

Why

The current Keys page only displays variables registered in OPTIONAL_ENV_VARS. That works for curated provider keys, but it makes valid user/plugin/tool environment variables invisible even though the backend can already save them.

This enhancement keeps the curated UX while allowing users to add project-specific or integration-specific keys from the WebUI.

Verification

• python3 -m py_compile hermes_cli/web_server.py
• cd web && source ~/.nvm/nvm.sh && nvm use 24 && npm run build

Note: the web build requires Node 20.19+ / 22.12+; Node 24 was used for verification.

[teknium1]

Implemented on main; closing via automated hermes-sweeper review.

• Commit 1c75e7c9d81230b8e1743e2ab626525c2231631f (feat(dashboard): list & add arbitrary custom .env keys on the Keys page) added the Custom Keys add-key form in web/src/pages/EnvPage.tsx:493 and is included in v2026.7.1.
• hermes_cli/web_server.py:5985 now returns arbitrary non-channel-managed on-disk .env keys as masked custom rows, while hermes_cli/web_server.py:6001 persists values through the existing guarded PUT /api/env route.
• tests/hermes_cli/test_env_custom_keys.py:31 covers custom-key listing and masking behavior.
• The linked discussion on feat(dashboard): allow creating new env keys from Keys tab #49288 identified this PR as the earlier duplicate; feat(dashboard): list & add arbitrary custom .env keys on the Keys page #54552 supplied the merged implementation.