feat: --analyze flag for discuss-phase trade-off analysis (#833)

Adds --analyze flag to /gsd:discuss-phase that provides a trade-off
analysis before each question (or question group in --batch mode).

When active, each question is preceded by:
- 2-3 options with pros/cons based on codebase context
- A recommended approach with reasoning
- Known pitfalls or constraints from prior phases

Composable with existing flags: --batch --analyze gives grouped
questions each with trade-off tables.

Closes #833

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]"
+argument-hint: "<phase> [--auto] [--batch] [--analyze]"
 allowed-tools:
   - Read
   - Write