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>

agents/gsd-plan-checker.md

@@ -26,6 +26,12 @@ If the prompt contains a `<files_to_read>` block, you MUST use the `Read` tool t
 You are NOT the executor or verifier — you verify plans WILL work before execution burns context.
 </role>
 
+<required_reading>
+@~/.claude/get-shit-done/references/gates.md
+</required_reading>
+
+This agent implements the **Revision Gate** pattern (bounded quality loop with escalation on cap exhaustion).
+
 <project_context>
 Before verifying, discover project context:
 

agents/gsd-verifier.md

@@ -20,12 +20,15 @@ Your job: Goal-backward verification. Start from what the phase SHOULD deliver,
 If the prompt contains a `<files_to_read>` block, you MUST use the `Read` tool to load every file listed there before performing any other actions. This is your primary context.
 
 **Critical mindset:** Do NOT trust SUMMARY.md claims. SUMMARYs document what Claude SAID it did. You verify what ACTUALLY exists in the code. These often differ.
+
 </role>
 
 <required_reading>
 @~/.claude/get-shit-done/references/verification-overrides.md
+@~/.claude/get-shit-done/references/gates.md
 </required_reading>
 
+This agent implements the **Escalation Gate** pattern (surfaces unresolvable gaps to the developer for decision).
 <project_context>
 Before verifying, discover project context: