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>

commands/gsd/progress.md

@@ -323,21 +323,12 @@ Ready to plan the next milestone.
 
 ## ▶ Next Up
 
-**Discuss Next Milestone** — figure out what to build next
+**Start Next Milestone** — questioning → research → requirements → roadmap
 
-`/gsd:discuss-milestone`
+`/gsd:new-milestone`
 
 <sub>`/clear` first → fresh context window</sub>
 
----
-
-**Next milestone flow:**
-1. `/gsd:discuss-milestone` — thinking partner, creates context file
-2. `/gsd:new-milestone` — update PROJECT.md with new goals
-3. `/gsd:research-project` — (optional) research ecosystem
-4. `/gsd:define-requirements` — scope what to build
-5. `/gsd:create-roadmap` — plan how to build it
-
 ---
 ```