docs: merge duplicate configuration docs into single guide

Merged 'Guild Isolation.md' and 'Per-Guild Configuration Implementation.md'
into a single comprehensive 'Per-Guild Configuration.md' document.

Changes:
- Consolidated redundant information from both documents
- Fixed conflicting information about which settings are global vs per-guild
- Removed outdated 'What Still Needs Work' sections (issues now fixed)
- Improved organization with clear sections for schema, implementation,
  commands, heuristics, testing, troubleshooting, and migration
- Added comprehensive data scope summary table
- Clarified that Persona, Logs Channel, Dry-run, etc. are per-guild
  (not global as incorrectly stated in old Guild Isolation doc)

Author: lukeocodes