refactor: unify milestone flow, remove deprecated commands

Consolidate /gsd:new-milestone to mirror /gsd:new-project:
- questioning → research (optional) → requirements → roadmap

BREAKING CHANGES:
- Remove /gsd:discuss-milestone (consolidated into new-milestone)
- Remove /gsd:create-roadmap (integrated into project/milestone flows)
- Remove /gsd:define-requirements (integrated into project/milestone flows)
- Remove /gsd:research-project (integrated into project/milestone flows)

Deleted files:
- commands/gsd/{discuss-milestone,create-roadmap,define-requirements,research-project}.md
- get-shit-done/workflows/{discuss-milestone,create-roadmap,define-requirements}.md
- get-shit-done/templates/milestone-context.md

Updated references in:
- progress.md, complete-milestone.md, help.md
- complete-milestone workflow, continuation-format.md
- questioning.md, requirements.md template, SUMMARY.md template
- gsd-project-researcher.md, gsd-roadmapper.md
- README.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,24 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Changed
+- **Unified `/gsd:new-milestone` flow** — Now mirrors `/gsd:new-project` with questioning → research → requirements → roadmap
+- **BREAKING:** Removed `/gsd:discuss-milestone` command (consolidated into `/gsd:new-milestone`)
+- **BREAKING:** Removed `/gsd:create-roadmap` command (integrated into project/milestone flows)
+- **BREAKING:** Removed `/gsd:define-requirements` command (integrated into project/milestone flows)
+- **BREAKING:** Removed `/gsd:research-project` command (integrated into project/milestone flows)
+- Roadmapper agent now references templates instead of inline structures
+
+### Removed
+- `commands/gsd/discuss-milestone.md`
+- `commands/gsd/create-roadmap.md`
+- `commands/gsd/define-requirements.md`
+- `commands/gsd/research-project.md`
+- `get-shit-done/workflows/discuss-milestone.md`
+- `get-shit-done/workflows/create-roadmap.md`
+- `get-shit-done/workflows/define-requirements.md`
+- `get-shit-done/templates/milestone-context.md`
+
 ## [1.5.30] - 2026-01-17
 
 ### Fixed