fix(checkpoints): restore rich documentation and fix continuation pattern

- execute-plan.md: Replace broken Task(resume=...) with fresh continuation agent
- execute-phase.md: Expand checkpoint handling with full continuation flow
- checkpoints.md: Restore ~500 lines of detailed examples and anti-patterns

Checkpoint presentation formats now documented for all three types.
Templates referenced: checkpoint-return.md, continuation-prompt.md

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

commands/gsd/execute-phase.md

@@ -160,9 +160,23 @@ All three run in parallel. Task tool blocks until all complete.
 <checkpoint_handling>
 Plans with `autonomous: false` in frontmatter have checkpoints:
 - Run in their assigned wave (can be parallel with other plans)
-- Pause at checkpoint, return to orchestrator
-- Orchestrator presents checkpoint to user
-- User responds, orchestrator resumes agent
+- Pause at checkpoint, return structured state to orchestrator
+- Orchestrator presents checkpoint to user (see execute-plan.md for presentation formats)
+- User responds, orchestrator spawns fresh continuation agent
+
+**Checkpoint flow:**
+1. Subagent returns `## CHECKPOINT REACHED` with Completed Tasks table
+2. Orchestrator parses return and presents rich formatted checkpoint
+3. User responds (approved/done/option selection)
+4. Orchestrator fills continuation-prompt.md template with state
+5. Spawns fresh agent: `Task(prompt=filled_continuation_template, subagent_type="general-purpose")`
+
+**Why fresh agent, not resume:**
+Task tool resume fails after multiple tool calls. Fresh agent with state handoff is the correct pattern.
+
+**Templates:**
+- `@~/.claude/get-shit-done/templates/checkpoint-return.md` - Subagent return format
+- `@~/.claude/get-shit-done/templates/continuation-prompt.md` - Fresh agent spawn template
 </checkpoint_handling>
 
 <deviation_rules>