fix: isolate profile state in separate directories

[domenkozar]

Each profile now gets its own state directory to prevent old state from persisting when switching profiles. This fixes the issue where environment variables, processes, and GC roots from one profile would leak into another.

• No profile: uses .devenv/ (backward compatible)
• With profiles: uses .devenv/profiles/<sorted-joined-names>/
• Example: --profile backend --profile frontend → .devenv/profiles/backend-frontend/

All state files are now profile-isolated:

• gc/shell, processes.pid, processes.log, input-paths.txt, etc.

Fixes #2165

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