enhancement: expose workspace_root on base_cli.Context

[codeforester]

Problem

Workspace-aware Python commands have to reach through ctx.user_config.workspace.root to find the configured workspace root. That value is typed and correct, but verbose for a high-use command concern.

Verified in lib/python/base_cli/context.py: Context exposes project_root directly, but not workspace_root. Verified in lib/python/base_cli/app.py: _create_context() loads user_config but does not derive a direct workspace-root field.

Proposal

Expose workspace_root: Path | None on base_cli.Context, derived from user_config.workspace.root during context creation.

Expected behavior:

• Context includes workspace_root: Path | None = None.
• _create_context() sets workspace_root=user_config.workspace.root.
• Workspace-oriented command code can use ctx.workspace_root directly.
• Existing ctx.user_config.workspace.root callers keep working.

Acceptance Criteria

• Unit tests cover ctx.workspace_root when user config has a workspace root.
• Unit tests cover ctx.workspace_root is None when no workspace root is configured.
• At least one workspace-oriented Python command or test fixture uses the convenience field where it improves clarity.
• lib/python/base_cli/README.md documents ctx.workspace_root alongside ctx.project_root.

Notes

Derived from the base_cli 1.1.0 follow-up review. This is one of the highest-impact items because workspace commands already treat the workspace root as a first-class concept.