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/COMMANDS.md

@@ -132,6 +132,45 @@ User acceptance testing with auto-diagnosis.
 
 ---
 
+### `/gsd:next`
+
+Automatically advance to the next logical workflow step. Reads project state and runs the appropriate command.
+
+**Prerequisites:** `.planning/` directory exists
+**Behavior:**
+- No project → suggests `/gsd:new-project`
+- Phase needs discussion → runs `/gsd:discuss-phase`
+- Phase needs planning → runs `/gsd:plan-phase`
+- Phase needs execution → runs `/gsd:execute-phase`
+- Phase needs verification → runs `/gsd:verify-work`
+- All phases complete → suggests `/gsd:complete-milestone`
+
+```bash
+/gsd:next                           # Auto-detect and run next step
+```
+
+---
+
+### `/gsd:session-report`
+
+Generate a session report with work summary, outcomes, and estimated resource usage.
+
+**Prerequisites:** Active project with recent work
+**Produces:** `.planning/reports/SESSION_REPORT.md`
+
+```bash
+/gsd:session-report                 # Generate post-session summary
+```
+
+**Report includes:**
+- Work performed (commits, plans executed, phases progressed)
+- Outcomes and deliverables
+- Blockers and decisions made
+- Estimated token/cost usage
+- Next steps recommendation
+
+---
+
 ### `/gsd:ship`
 
 Create PR from completed phase work with auto-generated body.