feat(cdt): add HANDOFF.md session artifact to wrap-up

[rube-de]

Part of #104

Context

CDT's auto-task wrap-up (auto-task.md:104) cleans up branch state: rm -rf ".claude/$BRANCH/". No session artifact is preserved. Each new session starts cold — decisions, context, and rationale from prior runs are lost.

Elite practice: every session ends with a structured handoff documenting decisions made, rationale, open questions, and context the next session needs. This compounds knowledge across sessions instead of restarting cold each time.

Task

Add a HANDOFF.md step to auto-task wrap-up (before cleanup) and dev-workflow wrap-up that writes a structured session artifact to .claude/files/.

Acceptance Criteria

• VERIFY: Auto-task writes HANDOFF.md to .claude/files/handoff-$TIMESTAMP.md before cleaning up branch state
• VERIFY: HANDOFF.md includes: task summary, key decisions with rationale, files changed, open questions, and what a future session should know
• VERIFY: HANDOFF.md is written before rm -rf ".claude/$BRANCH/" (line 104)
• VERIFY: Dev-workflow wrap-up (line 290) also writes HANDOFF.md (for non-auto-task runs)
• VERIFY: HANDOFF.md references the plan path and dev report path

Implementation Guide

Files to Modify

• plugins/cdt/commands/auto-task.md — Add HANDOFF.md step before cleanup (before line 104)
• plugins/cdt/skills/cdt/references/dev-workflow.md — Add HANDOFF.md step before wrap-up (before line 299)

Approach

1. In auto-task.md, Wrap Up section (before line 104 cleanup), add:

   5a. Write session handoff to `.claude/files/handoff-$TIMESTAMP.md`:
   
       # Session Handoff
   
       **Task**: [original task description]
       **Date**: [date]  **Branch**: [branch name]
   
       ## Decisions Made
       [Key architectural and implementation decisions with rationale — WHY, not just WHAT]
   
       ## Files Changed
       [List of all modified/created files]
   
       ## Plan & Report
       - Plan: [plan path]
       - Dev Report: [dev report path]
   
       ## Open Questions
       [Anything unresolved, deferred, or uncertain]
   
       ## Context for Next Session
       [What a future session working in this area should know]
   

2. In dev-workflow.md, add the same template before the Wrap Up section (line 290), so non-auto-task flows also produce handoffs
3. Ensure the handoff uses $TIMESTAMP from Step 2 for naming consistency

Patterns to Follow

• The dev report template (dev-workflow.md:260-288) follows a similar structured format
• .claude/files/ is already used for dev reports (dev-workflow.md:253)

Scope

In Scope

• Adding HANDOFF.md write step to auto-task.md and dev-workflow.md
• Defining the handoff template

Out of Scope

• Reading previous handoffs at session start (future enhancement)
• Adding handoff to plan-workflow.md (plan artifacts are the handoff for the plan phase)

Dependencies

• Blocked by: None
• Blocks: None

Definition of Done

• Implementation complete
• bun scripts/validate-plugins.mjs passes
• PR approved and merged
• Epic checklist updated