feat(10-03): create execute-phase command

- New slash command for parallel phase execution
- References execute-phase.md workflow
- Requires phase number as argument
- Uses parallel agent spawning for concurrent plans

commands/gsd/execute-phase.md

@@ -0,0 +1,56 @@
+---
+name: gsd:execute-phase
+description: Execute all plans in a phase with intelligent parallelization
+argument-hint: "<phase-number>"
+allowed-tools:
+  - Read
+  - Write
+  - Edit
+  - Bash
+  - Glob
+  - Grep
+  - Task
+  - TaskOutput
+  - AskUserQuestion
+---
+
+<objective>
+Execute all unexecuted plans in a phase with parallel agent spawning.
+
+Analyzes plan dependencies to identify independent plans that can run concurrently.
+Spawns background agents for parallel execution, orchestrates commits after completion.
+
+Use this command when:
+- Phase has 2+ unexecuted plans
+- Want "walk away, come back to completed work" execution
+- Plans have clear dependency boundaries
+</objective>
+
+<execution_context>
+@~/.claude/get-shit-done/workflows/execute-phase.md
+@~/.claude/get-shit-done/templates/summary.md
+</execution_context>
+
+<context>
+Phase number: $ARGUMENTS (required)
+
+@.planning/STATE.md
+@.planning/config.json
+</context>
+
+<process>
+1. Validate phase exists in roadmap
+2. Find all PLAN.md files without matching SUMMARY.md
+3. If 0 or 1 plans: suggest /gsd:execute-plan instead
+4. If 2+ plans: follow execute-phase.md workflow
+5. Monitor parallel agents until completion
+6. Present results and next steps
+</process>
+
+<success_criteria>
+- [ ] All independent plans executed in parallel
+- [ ] Dependent plans executed after dependencies complete
+- [ ] All SUMMARY.md files created
+- [ ] Commits created by orchestrator
+- [ ] Phase progress updated
+</success_criteria>