Add process-level secret injection (vaultix exec)

[Zayan-Mohamed]

• Vaultix currently only supports extracting secrets to disk. This exposes secrets to filesystem leaks, backups, and accidental commits.

• We need a mode that decrypts secrets into memory, injects them into a child process as environment variables, and wipes them after execution.

• Proposed UX

vaultix exec --env AWS_KEY=aws_key.txt --env DB_PASS=db.txt -- myapp

This should:

• Decrypt secrets in memory
• Set them as environment variables
• Execute the target command
• Zero memory after process exit

Acceptance Criteria

• No secret data written to disk
• Secrets available only to the spawned process
• Memory wiped after execution
• Works cross-platform