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/CONFIGURATION.md

@@ -30,7 +30,10 @@ GSD stores project settings in `.planning/config.json`. Created during `/gsd:new
     "ui_safety_gate": true,
     "node_repair": true,
     "node_repair_budget": 2,
-    "research_before_questions": false
+    "research_before_questions": false,
+    "discuss_mode": "discuss",
+    "skip_discuss": false,
+    "text_mode": false
   },
   "hooks": {
     "context_warnings": true,
@@ -97,6 +100,9 @@ All workflow toggles follow the **absent = enabled** pattern. If a key is missin
 | `workflow.node_repair` | boolean | `true` | Autonomous task repair on verification failure |
 | `workflow.node_repair_budget` | number | `2` | Max repair attempts per failed task |
 | `workflow.research_before_questions` | boolean | `false` | Run research before discussion questions instead of after |
+| `workflow.discuss_mode` | string | `'discuss'` | Controls how `/gsd:discuss-phase` gathers context. `'discuss'` (default) asks questions one-by-one. `'assumptions'` reads the codebase first, generates structured assumptions with confidence levels, and only asks you to correct what's wrong. Added in v1.28 |
+| `workflow.skip_discuss` | boolean | `false` | When `true`, `/gsd:autonomous` bypasses the discuss-phase entirely, writing minimal CONTEXT.md from the ROADMAP phase goal. Useful for projects where developer preferences are fully captured in PROJECT.md/REQUIREMENTS.md. Added in v1.28 |
+| `workflow.text_mode` | boolean | `false` | Replaces AskUserQuestion TUI menus with plain-text numbered lists. Required for Claude Code remote sessions (`/rc` mode) where TUI menus don't render. Can also be set per-session with `--text` flag on discuss-phase. Added in v1.28 |
 
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