Create workspace base directories during installation

[tcarn]

Problem

install.sh writes COPILOT_WORKSPACE_BASE and CLAUDE_WORKSPACE_BASE to ~/.config/remote-agent-stack/config, but does not create those directories.

The first documented launch is ca alpha, yet bin/agent currently checks that the configured workspace base already exists and exits if it does not:

COPILOT workspace base not available: /Users/<user>/copilot-workspace

This means a fresh installation requires an undocumented manual mkdir -p step before the first launch.

Expected behavior

After a successful install, the configured workspace roots should exist so the documented first launch works immediately:

ca alpha

The wrapper can then create the per-agent directory (agent-alpha) as it does today.

Proposed change

After resolving COPILOT_WORKSPACE_BASE_RESOLVED and CLAUDE_WORKSPACE_BASE_RESOLVED, create both directories with appropriate user-private permissions, for example:

mkdir -p "$COPILOT_WORKSPACE_BASE_RESOLVED" \
         "$CLAUDE_WORKSPACE_BASE_RESOLVED"
chmod 700 "$COPILOT_WORKSPACE_BASE_RESOLVED" \
          "$CLAUDE_WORKSPACE_BASE_RESOLVED"

The implementation should preserve existing directories and work for custom paths, Dropbox paths, and reruns. If directory creation fails, the installer should fail clearly rather than writing a configuration that cannot be used.

Acceptance criteria

• A fresh interactive install creates both configured workspace roots.
• Custom --copilot-workspace-base and --claude-workspace-base paths are handled.
• Existing directories and contents are preserved on rerun.
• A fresh install can run ca alpha without a manual directory-creation step.
• Installer tests cover successful creation and a creation failure.
• README setup instructions no longer imply an extra manual mkdir is needed.