[feat]: Shell mode: add pseudo-cwd so cd affects subsequent commands

[JessyTsui]

What feature would you like to see?

Background / Problem

In shell mode, each command is executed independently and cd does not persist,
which makes shell usage cumbersome.

Expected Behavior

Shell mode should maintain a pseudo-cwd so that cd <dir> affects subsequent
commands.

Proposed Approach

Track self._shell_cwd in the shell layer; pass it as cwd to subprocess calls;
handle cd internally to update the pseudo-cwd.

Scope / Impact

• Only affects shell mode
• Does not change the agent work directory or context

Acceptance Criteria

• After cd subdir, pwd shows subdir
• Relative paths resolve against pseudo-cwd
• Clear error when directory does not exist

Additional information

No response

[Br1an67]

I've submitted a PR to address this: #1526

Implementation: Added a _shell_cwd pseudo-cwd to the Shell class that tracks the working directory across shell mode commands. The cd command now updates this pseudo-cwd instead of showing a warning, and all subsequent commands run with cwd=self._shell_cwd.

Supports bare cd (→ home), relative/absolute paths, ~ expansion, and clear error messages for invalid targets. Does not affect the agent working directory.