feat(discuss): add --chain flag for interactive discuss with auto plan+execute

Adds --chain flag to /gsd:discuss-phase that provides the middle ground
between fully manual and fully automatic workflows:

  /gsd:discuss-phase 5 --chain

  - Discussion is fully interactive (user answers questions)
  - After context is captured, auto-advances to plan → execute
  - Same pipeline as --auto, but without auto-answering

This addresses the community request for per-phase automation where
users want to control discuss decisions but skip manual advancement
between plan and execute steps.

Workflow: discuss (interactive) → plan (auto) → execute (auto)

Changes:
- Workflow: --chain flag triggers auto_advance without auto-answering
- Workflow: chain flag synced alongside --auto in ephemeral config
- Workflow: next-phase suggestion preserves --chain vs --auto
- Command: argument-hint and description updated
- Success criteria updated

Closes #1327

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

commands/gsd/discuss-phase.md

@@ -1,7 +1,7 @@
 ---
 name: gsd:discuss-phase
-description: Gather phase context through adaptive questioning before planning. Use --auto to skip interactive questions (Claude picks recommended defaults).
-argument-hint: "<phase> [--auto] [--batch] [--analyze] [--text]"
+description: Gather phase context through adaptive questioning before planning. Use --auto to skip interactive questions (Claude picks recommended defaults). Use --chain for interactive discuss followed by automatic plan+execute.
+argument-hint: "<phase> [--auto] [--chain] [--batch] [--analyze] [--text]"
 allowed-tools:
   - Read
   - Write