fix(agent-sdk): prevent generateKeys from overwriting .env

[sashaphmn]

Prevent generateKeys from accidentally overwriting an existing .env file. It now refuses to overwrite it by default and only does so with an explicit --force, with a warning shown first.

Note

Prevent generateKeys from overwriting an existing .env file without --force

The script now uses wx (create-only) file semantics and exits with code 1 if a .env file already exists. Passing --force switches to w mode, overwrites the file, and prints a warning. The INIT_CWD presence check is also moved before key generation so no keys are produced if the environment is invalid.

^{Macroscope summarized a8d2684.}