[FEATURE] Add a "Prompt" / instructions field to cloud environment configuration

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Cloud environment setup (claude.ai/code → Add cloud environment) only has Environment variables and Setup script. There's no field for freeform instructions specific to that environment — so context Claude should have about it (what it's for, non-secret operational notes, how to treat its env vars/services) has to be smuggled into setup-script comments or a repo-wide CLAUDE.md.

Proposed Solution

Add a Prompt/Instructions field to the environment config, injected into session context at startup for any session run against that environment — scoped to the environment, not the repo or the account.

Alternative Solutions

• Have the Setup script write a CLAUDE.md file into the environment's filesystem — doesn't work reliably because the cloud environment's filesystem layout (where the repo is checked out, whether a CLAUDE.md written there even gets picked up) isn't documented, so there's no dependable path to target.
• Put notes in the Setup script as comments — not reliably treated as instructions.
• Commit a CLAUDE.md to the repo — applies repo-wide, not per-environment.

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

• A GITHUB_TOKEN env var is set to authenticate private package installs, but Claude doesn't reliably pick it up and wire it into .npmrc/pip.conf on its own; a Prompt field could just state that requirement so it's always applied.
• A multi-repo environment has a required launch order and cross-repo dependencies (e.g. start the backend first, wait for it, then the frontend); a Prompt field could state that order once instead of it being re-explained every session.

Additional Context

_No response_