fix(config): preserve spaces in config values

[jbetala7]

Summary

• Preserve everything after key: when gstack-config get reads config values, instead of taking only the second whitespace-delimited field
• Reuse the same reader for gstack-config list active values
• Add a regression test for workspace_root values containing spaces

Fixes #1782

Root cause

gstack-config get used awk '{print $2}', which silently truncated values like workspace_root: /tmp/Conductor Workspaces to /tmp/Conductor. gstack-next-version consumes workspace_root for sibling worktree scanning, so configured roots with spaces were ignored.

Tests

• bun test test/explain-level-config.test.ts test/docs-config-keys.test.ts
• bun test test/gstack-next-version.test.ts test/brain-sync.test.ts test/explain-level-config.test.ts
• bash -n bin/gstack-config
• git diff --check
• Direct repro: gstack-config set workspace_root "/tmp/Conductor Workspaces" then get workspace_root returns the full path

Collision checks

Searched open and closed issues/PRs for workspace_root space, gstack-config config.yaml space, and open PRs touching gstack-config. No existing issue or canonical PR covered this truncation bug.