feat: add /gsd:next command for automatic workflow advancement (#927) (#1167)

Adds a zero-friction command that detects the current project state and
automatically invokes the next logical workflow step:

- No phases → discuss first phase
- Phase has no context → discuss
- Phase has context but no plans → plan
- Phase has plans but incomplete → execute
- All plans complete → verify and complete phase
- All phases complete → complete milestone
- Paused → resume work

No arguments needed — reads STATE.md, ROADMAP.md, and phase directories
to determine progression. Designed for multi-project workflows.

Closes #927

commands/gsd/next.md

@@ -0,0 +1,24 @@
+---
+name: gsd:next
+description: Automatically advance to the next logical step in the GSD workflow
+allowed-tools:
+  - Read
+  - Bash
+  - Grep
+  - Glob
+  - SlashCommand
+---
+<objective>
+Detect the current project state and automatically invoke the next logical GSD workflow step.
+No arguments needed — reads STATE.md, ROADMAP.md, and phase directories to determine what comes next.
+
+Designed for rapid multi-project workflows where remembering which phase/step you're on is overhead.
+</objective>
+
+<execution_context>
+@~/.claude/get-shit-done/workflows/next.md
+</execution_context>
+
+<process>
+Execute the next workflow from @~/.claude/get-shit-done/workflows/next.md end-to-end.
+</process>