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/references/gates.md

@@ -0,0 +1,70 @@
+# Gates Taxonomy
+
+Canonical gate types used across GSD workflows. Every validation checkpoint maps to one of these four types.
+
+---
+
+## Gate Types
+
+### Pre-flight Gate
+**Purpose:** Validates preconditions before starting an operation.
+**Behavior:** Blocks entry if conditions unmet. No partial work created.
+**Recovery:** Fix the missing precondition, then retry.
+**Examples:**
+- Plan-phase checks for REQUIREMENTS.md before planning
+- Execute-phase validates PLAN.md exists before execution
+- Discuss-phase confirms phase exists in ROADMAP.md
+
+### Revision Gate
+**Purpose:** Evaluates output quality and routes to revision if insufficient.
+**Behavior:** Loops back to producer with specific feedback. Bounded by iteration cap.
+**Recovery:** Producer addresses feedback; checker re-evaluates. The loop also escalates early if issue count does not decrease between consecutive iterations (stall detection). After max iterations, escalates unconditionally.
+**Examples:**
+- Plan-checker reviewing PLAN.md (max 3 iterations)
+- Verifier checking phase deliverables against success criteria
+
+### Escalation Gate
+**Purpose:** Surfaces unresolvable issues to the developer for a decision.
+**Behavior:** Pauses workflow, presents options, waits for human input.
+**Recovery:** Developer chooses action; workflow resumes on selected path.
+**Examples:**
+- Revision loop exhausted after 3 iterations
+- Merge conflict during worktree cleanup
+- Ambiguous requirement needing clarification
+
+### Abort Gate
+**Purpose:** Terminates the operation to prevent damage or waste.
+**Behavior:** Stops immediately, preserves state, reports reason.
+**Recovery:** Developer investigates root cause, fixes, restarts from checkpoint.
+**Examples:**
+- Context window critically low during execution
+- STATE.md in error state blocking /gsd-next
+- Verification finds critical missing deliverables
+
+---
+
+## Gate Matrix
+
+| Workflow | Phase | Gate Type | Artifacts Checked | Failure Behavior |
+|----------|-------|-----------|-------------------|------------------|
+| plan-phase | Entry | Pre-flight | REQUIREMENTS.md, ROADMAP.md | Block with missing-file message |
+| plan-phase | Step 12 | Revision | PLAN.md quality | Loop to planner (max 3) |
+| plan-phase | Post-revision | Escalation | Unresolved issues | Surface to developer |
+| execute-phase | Entry | Pre-flight | PLAN.md | Block with missing-plan message |
+| execute-phase | Completion | Revision | SUMMARY.md completeness | Re-run incomplete tasks |
+| verify-work | Entry | Pre-flight | SUMMARY.md | Block with missing-summary |
+| verify-work | Evaluation | Escalation | Failed criteria | Surface gaps to developer |
+| next | Entry | Abort | Error state, checkpoints | Stop with diagnostic |
+
+---
+
+## Implementing Gates
+
+Use this taxonomy when designing or auditing workflow validation points:
+
+- **Pre-flight** gates belong at workflow entry points. They are cheap, deterministic checks that prevent wasted work. If you can verify a precondition with a file-existence check or a config read, use a pre-flight gate.
+- **Revision** gates belong after a producer step where quality varies. Always pair them with an iteration cap to prevent infinite loops. The cap should reflect the cost of each iteration -- expensive operations get fewer retries.
+- **Escalation** gates belong wherever automated resolution is impossible or ambiguous. They are the safety valve between revision loops and abort. Present the developer with clear options and enough context to decide.
+- **Abort** gates belong at points where continuing would cause damage, waste significant resources, or produce meaningless output. They should preserve state so work can resume after the root cause is fixed.
+
+**Selection heuristic:** Start with pre-flight. If the check happens after work is produced, it is a revision gate. If the revision loop cannot resolve the issue, escalate. If continuing is dangerous, abort.

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>

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

@@ -9,6 +9,7 @@ Read all files referenced by the invoking prompt's execution_context before star
 @~/.claude/get-shit-done/references/revision-loop.md
 @~/.claude/get-shit-done/references/gate-prompts.md
 @~/.claude/get-shit-done/references/agent-contracts.md
+@~/.claude/get-shit-done/references/gates.md
 </required_reading>
 
 <available_agent_types>