Replace split_whitespace with shlex for shell-aware argument parsing

[aniongithub]

Summary

Replaces naive split_whitespace() argument splitting with the shlex crate for proper shell-aware parsing across all tools. Also replaces shell-escape with shlex for quoting (one crate for both directions).

Fixes #10, fixes #15

Changes

devcontainer_exec — always wrap in sh -c

The agent passes commands exactly like it would on the host:

devcontainer_exec(workspace_folder: "/path", command: "cargo build")
devcontainer_exec(workspace_folder: "/path", command: "cd /foo && npm test")
devcontainer_exec(workspace_folder: "/path", command: "git commit -m \"my message\"")

We handle the hard bits — no need for the agent to split command/args or worry about quoting. This matches how devpod_ssh and codespaces_ssh already work.

5 other tools — shlex::split() for CLI flags

devcontainer_up, devcontainer_build, devpod_up, devpod_build, devpod_provider_add now use shlex::split() instead of split_whitespace() so quoted values in extra args are preserved.

file_ops — shlex::try_quote() replaces shell_escape

Removes the shell-escape dependency. shlex handles both splitting and quoting.

Why

The design goal is simplicity for the agent: pass a command string, we handle the rest. Imposing quoting/splitting rules on agents makes it harder for smaller models to work correctly.