feat(references): add gates taxonomy with 4 canonical gate types (#1781)

* feat(references): add gates taxonomy with 4 canonical gate types

Define pre-flight, revision, escalation, and abort gates as the
canonical validation checkpoint types used across GSD workflows.
Includes a gate matrix mapping each workflow phase to its gate type,
checked artifacts, and failure behavior. Cross-referenced from
plan-phase and execute-phase workflows.

Closes #1715

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(agents): add gates.md reference to plan-checker and verifier per approved scope (#1715)

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

* fix(agents): move gates.md to required_reading blocks and add stall detection (#1715)

- Move gates.md @-reference from <role> prose into <required_reading> blocks
  in gsd-plan-checker.md and gsd-verifier.md so it loads as context
- Add stall-detection to Revision Gate recovery description
- Fix /gsd-next → next for consistent workflow naming in Gate Matrix
- Update tests to verify required_reading placement and stall detection

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

---------

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

get-shit-done/workflows/execute-phase.md

@@ -28,6 +28,7 @@ Read STATE.md before any operation to load project context.
 
 @~/.claude/get-shit-done/references/agent-contracts.md
 @~/.claude/get-shit-done/references/context-budget.md
+@~/.claude/get-shit-done/references/gates.md
 </required_reading>
 
 <available_agent_types>