docs: v1.28 release documentation update

Add documentation for all new features merged since v1.27:

- Forensics command (/gsd:forensics) — post-mortem workflow investigation
- Milestone Summary (/gsd:milestone-summary) — project summary for onboarding
- Workstream Namespacing (/gsd:workstreams) — parallel milestone work
- Manager Dashboard (/gsd:manager) — interactive phase command center
- Assumptions Discussion Mode (workflow.discuss_mode) — codebase-first context
- UI Phase Auto-Detection — surface /gsd:ui-phase for UI-heavy projects
- Multi-Runtime Installer Selection — select multiple runtimes interactively

Updated files:
- README.md: new commands, config keys, assumptions mode callout
- docs/COMMANDS.md: 4 new command entries with full syntax
- docs/FEATURES.md: 7 new feature entries (#49-#55) with requirements
- docs/CONFIGURATION.md: 3 new workflow config keys
- docs/AGENTS.md: 2 new agents, count 15→18
- docs/USER-GUIDE.md: assumptions mode, forensics, workstreams, non-Claude runtimes
- docs/README.md: updated index with discuss-mode doc link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/README.md

@@ -14,9 +14,11 @@ Comprehensive documentation for the Get Shit Done (GSD) framework — a meta-pro
 | [Agent Reference](AGENTS.md) | Contributors, advanced users | All 15 specialized agents — roles, tools, spawn patterns |
 | [User Guide](USER-GUIDE.md) | All users | Workflow walkthroughs, troubleshooting, and recovery |
 | [Context Monitor](context-monitor.md) | All users | Context window monitoring hook architecture |
+| [Discuss Mode](workflow-discuss-mode.md) | All users | Assumptions vs interview mode for discuss-phase |
 
 ## Quick Links
 
+- **What's new in v1.28:** Forensics, milestone summary, workstreams, assumptions mode, UI auto-detect, manager dashboard
 - **Getting started:** [README](../README.md) → install → `/gsd:new-project`
 - **Full workflow walkthrough:** [User Guide](USER-GUIDE.md)
 - **All commands at a glance:** [Command Reference](COMMANDS.md)