feat: add exec command for running containers

[kacpersaw]

Summary

Adds ocdev exec to run commands inside an existing container without opening an interactive shell or defining a recipe task.

ocdev exec demo -- uname -a
ocdev exec demo --cwd /home/dev/workspace/app -- npm test
printf 'hello\n' | ocdev exec demo -- cat

What was added

• Execution as dev, with /home/dev as the default directory and optional --cwd.
• Direct argument forwarding after --, including the program’s own flags.
• Raw stdin, separate stdout/stderr, and command exit-code forwarding.
• Cancellation handling, documentation, and make test-exec.

There is no ocdev-level --json, implicit shell, pseudo-terminal, or command/output logging. Interactive sessions still use ocdev shell.

Behavior and safety

Containers must already be running. Exec holds the environment lock and checks pinned UUIDs for recipe-managed containers.

Interruption cleans up the local Incus client, but does not guarantee the guest command has stopped. The abbreviation ex is now ambiguous with export; use exec explicitly.

Testing

• make test passed: 115 unit/integration tests plus version regressions.
• Includes 13 exec test groups covering arguments, binary streams, stdin, exit codes, locking, UUID checks, and cancellation.
• Verification used a clean HOME and a PATH without real Incus or Python. No live container tests were run.