docs: fix brownfield flow, remove deprecated resume-task

- Fix brownfield example to use execute-phase instead of execute-plan
- Remove /gsd:resume-task command (relies on broken Task resume)
- Checkpoint continuation now uses fresh agents, not resume

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

commands/gsd/resume-task.md

@@ -1,87 +0,0 @@
----
-name: gsd:resume-task
-description: Resume an interrupted subagent execution
-argument-hint: "[agent-id]"
-allowed-tools:
-  - Read
-  - Write
-  - Edit
-  - Bash
-  - Task
-  - AskUserQuestion
----
-
-<objective>
-Resume an interrupted subagent execution using the Task tool's resume parameter.
-
-When a session ends mid-execution, subagents may be left in an incomplete state. This command allows users to continue that work without starting over.
-
-Uses the agent ID tracking infrastructure from execute-plan to identify and resume agents.
-</objective>
-
-<execution_context>
-@~/.claude/get-shit-done/workflows/resume-task.md
-</execution_context>
-
-<context>
-Agent ID: $ARGUMENTS (optional - defaults to most recent)
-
-**Load project state:**
-@.planning/STATE.md
-
-**Load agent tracking:**
-@.planning/current-agent-id.txt
-@.planning/agent-history.json
-</context>
-
-<process>
-1. Check .planning/ directory exists (error if not)
-2. Parse agent ID from arguments or current-agent-id.txt
-3. Validate agent exists in history and is resumable
-4. Check for file conflicts since spawn
-5. Follow resume-task.md workflow:
-   - Update agent status to "interrupted"
-   - Resume via Task tool resume parameter
-   - Update history on completion
-   - Clear current-agent-id.txt
-</process>
-
-<usage>
-**Resume most recent interrupted agent:**
-```
-/gsd:resume-task
-```
-
-**Resume specific agent by ID:**
-```
-/gsd:resume-task agent_01HXYZ123
-```
-
-**Find available agents to resume:**
-Check `.planning/agent-history.json` for entries with status "spawned" or "interrupted".
-</usage>
-
-<error_handling>
-**No agent to resume:**
-- current-agent-id.txt empty or missing
-- Solution: Run /gsd:progress to check project status
-
-**Agent already completed:**
-- Agent finished successfully, nothing to resume
-- Solution: Continue with next plan
-
-**Agent not found:**
-- Provided ID not in history
-- Solution: Check agent-history.json for valid IDs
-
-**Resume failed:**
-- Agent context expired or invalidated
-- Solution: Start fresh with /gsd:execute-plan
-</error_handling>
-
-<success_criteria>
-- [ ] Agent resumed via Task tool resume parameter
-- [ ] Agent-history.json updated with completion
-- [ ] current-agent-id.txt cleared
-- [ ] User informed of result
-</success_criteria>