feat: add in-memory environment injection and process execution command

[hammadmajid]

Problem

In shared developer machines, CI runners, or strict compliance environments, writing plaintext secrets to a physical .env file on disk introduces security risks of accidental exposure or leakage into logs/backups.

Proposed Solution

Introduce exenv exec -- <command> which resolves environment variables from templates, existing files, and shell sessions in-memory and injects them directly into a spawned child process without writing a file to disk.

Expected Behavior

• Running exenv exec -- npm run dev loads the configuration into memory, sets process environment variables, and executes the command.
• Child process inherits stdout, stderr, stdin, and exit codes cleanly.
• No .env file is written or left on the filesystem.

Acceptance Criteria

• Child process signals (SIGINT, SIGTERM) are forwarded properly.
• Exit code of the child process is propagated accurately to the host terminal.
• No sensitive data is leaked to temporary disk paths.