docs: comprehensive v1.26 release documentation update (#1187)

Updates all docs to reflect v1.26.0 features and changes:

README.md:
- Add /gsd:ship and /gsd:next to command tables
- Add /gsd:session-report to Session section
- Update workflow to show ship step and auto-advance
- Update inherit profile description for non-Anthropic providers

docs/COMMANDS.md:
- Add /gsd:next command reference with full state detection logic
- Add /gsd:session-report command reference with report contents

docs/FEATURES.md:
- Add Auto-Advance (Next) feature (#14)
- Add Cross-Phase Regression Gate feature (#20)
- Add Requirements Coverage Gate feature (#21)
- Add Session Reporting feature (#24)
- Fix all section numbering (was broken with duplicates)
- Update inherit profile to mention non-Anthropic providers
- Renumber all 39 features consistently

docs/USER-GUIDE.md:
- Add /gsd:ship to workflow diagram
- Add /gsd:next and /gsd:session-report to command tables
- Add HANDOFF.json and reports/ to file structure
- Add troubleshooting for non-Anthropic model providers
- Add recovery entries for session-report and next
- Update example workflow to include ship and session-report

docs/CONFIGURATION.md:
- Update inherit profile to mention non-Anthropic providers

docs/CONFIGURATION.md

@@ -246,7 +246,7 @@ Valid override values: `opus`, `sonnet`, `haiku`, `inherit`
 | `quality` | Opus for all decision-making, Sonnet for verification | Quota available, critical architecture work |
 | `balanced` | Opus for planning only, Sonnet for everything else | Normal development (default) |
 | `budget` | Sonnet for code-writing, Haiku for research/verification | High-volume work, less critical phases |
-| `inherit` | All agents use current session model | Dynamic model switching (OpenCode `/model`) |
+| `inherit` | All agents use current session model | Dynamic model switching, **non-Anthropic providers** (OpenRouter, local models) |
 
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