docs(readme): align manual uninstall with --keep-state split

[mvanhorn]

Summary

The bin/gstack-uninstall script already supports --keep-state, which removes skills but preserves ~/.gstack/ (config, analytics, sessions, project history, installation-id). The manual-removal block in README didn't reflect that split: it dropped rm -rf ~/.gstack inline as step 4 alongside the skill cleanup, so users following the manual path always lost their state even when the scripted path lets them keep it.

Reported in #1131.

Fix

• Label the numbered block as the --keep-state equivalent.
• Move the ~/.gstack purge into an "Also purge gstack data (optional)" subsection after the block, so users can decide whether to run it.

No script changes. Same behaviour is available, just documented as two choices instead of one.

Diff

+ Mirrors `gstack-uninstall --keep-state` (preserves `~/.gstack/` data):

  ```bash
  # 1. Stop browse daemons
  ...
  # 3. Remove gstack
  rm -rf ~/.claude/skills/gstack

- # 4. Remove global state
- rm -rf ~/.gstack
-
- # 5. Remove integrations ...
+ # 4. Remove integrations ...
  ...

• Also purge gstack data (optional)

• ~/.gstack/ holds config, analytics, sessions, project history, and the
• installation-id. Run this only if you want a clean slate (equivalent to
• gstack-uninstall without --keep-state):
• rm -rf ~/.gstack

Fixes #1131