feat(next): add hard stop safety gates and consecutive-call guard (#1784)

Add three hard-stop checks to /gsd-next that prevent blind advancement:
1. Unresolved .continue-here.md checkpoint from a previous session
2. Error/failed state in STATE.md
3. Unresolved FAIL items in VERIFICATION.md

Also add a consecutive-call budget guard that prompts after 6
consecutive /gsd-next calls, preventing runaway automation loops.

All gates are bypassed with --force (prints a one-line warning).
Gates run in order and exit on the first hit to give clear,
actionable diagnostics.

Closes #1732

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

commands/gsd/next.md

@@ -13,6 +13,8 @@ Detect the current project state and automatically invoke the next logical GSD w
 No arguments needed — reads STATE.md, ROADMAP.md, and phase directories to determine what comes next.
 
 Designed for rapid multi-project workflows where remembering which phase/step you're on is overhead.
+
+Supports `--force` flag to bypass safety gates (checkpoint, error state, verification failures).
 </objective>
 
 <execution_context>