feat: add workflow.discuss_mode assumptions config (#637)

Add codebase-first assumption-driven alternative to the interview-style
discuss-phase. New `workflow.discuss_mode: "assumptions"` config routes
to a separate workflow that spawns a gsd-assumptions-analyzer agent to
read 5-15 codebase files, surface assumptions with evidence, and ask
only for corrections (~2-4 interactions vs ~15-20).

- New gsd-assumptions-analyzer agent for deep codebase analysis
- New discuss-phase-assumptions.md workflow (15 steps)
- Command-level routing via dual @reference + process gate
- Identical CONTEXT.md output — downstream agents unaffected
- Existing discuss-phase.md workflow untouched (zero diff)
- Mode-aware plan-phase gate and progress display
- User documentation and integration tests
- Update agent count and list in copilot-install tests (17 → 18)

Closes #637

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

get-shit-done/workflows/progress.md

@@ -18,6 +18,10 @@ if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi
 
 Extract from init JSON: `project_exists`, `roadmap_exists`, `state_exists`, `phases`, `current_phase`, `next_phase`, `milestone_version`, `completed_count`, `phase_count`, `paused_at`, `state_path`, `roadmap_path`, `project_path`, `config_path`.
 
+```bash
+DISCUSS_MODE=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.discuss_mode 2>/dev/null || echo "discuss")
+```
+
 If `project_exists` is false (no `.planning/` directory):
 
 ```
@@ -99,6 +103,7 @@ Present:
 
 **Progress:** {PROGRESS_BAR}
 **Profile:** [quality/balanced/budget/inherit]
+**Discuss mode:** {DISCUSS_MODE}
 
 ## Recent Work
 - [Phase X, Plan Y]: [what was accomplished - 1 line from summary-extract]