task: promote validTransitions to package-level var; fix ADR

Hoists the map out of ValidTransition so it's not reallocated on every
call. Adds missing CANCELLED→QUEUED and BUDGET_EXCEEDED→QUEUED entries
to the ADR transition table to match the implemented state machine.

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

3 files changed, 25 insertions, 17 deletions

diff --git a/docs/adr/002-task-state-machine.md b/docs/adr/002-task-state-machine.md
index 1d41619..310c337 100644
--- a/docs/adr/002-task-state-machine.md
+++ b/docs/adr/002-task-state-machine.md
@@ -23,7 +23,7 @@ execution (subprocess), user interaction (review, Q&A), retries, and cancellatio
 | `BUDGET_EXCEEDED` | Exceeded `max_budget_usd` (terminal) |
 | `BLOCKED` | Agent paused and wrote a question file; awaiting user answer |
 
-Terminal states with no outgoing transitions: `COMPLETED`, `CANCELLED`, `BUDGET_EXCEEDED`.
+True terminal state (no outgoing transitions): `COMPLETED`. All other non-success states (`CANCELLED`, `FAILED`, `TIMED_OUT`, `BUDGET_EXCEEDED`) may transition back to `QUEUED` to restart or retry.
 
 ## State Transition Diagram
 
@@ -77,6 +77,8 @@ Terminal states with no outgoing transitions: `COMPLETED`, `CANCELLED`, `BUDGET_
 | `READY` | `PENDING` | `POST /api/tasks/{id}/reject` (with optional comment) |
 | `FAILED` | `QUEUED` | Retry (manual re-run via `POST /api/tasks/{id}/run`) |
 | `TIMED_OUT` | `QUEUED` | `POST /api/tasks/{id}/resume` (resumes with session ID) |
+| `CANCELLED` | `QUEUED` | Restart (manual re-run via `POST /api/tasks/{id}/run`) |
+| `BUDGET_EXCEEDED` | `QUEUED` | Retry (manual re-run via `POST /api/tasks/{id}/run`) |
 | `BLOCKED` | `QUEUED` | `POST /api/tasks/{id}/answer` (resumes with user answer) |
 | `BLOCKED` | `READY` | All subtasks reached `COMPLETED` (parent task unblocked by subtask completion watcher) |
 
diff --git a/internal/task/task.go b/internal/task/task.go
index c0aa036..9968b15 100644
--- a/internal/task/task.go
+++ b/internal/task/task.go
@@ -52,10 +52,10 @@ type Task struct {
 	ID           string       `yaml:"id"            json:"id"`
 	ParentTaskID string       `yaml:"parent_task_id" json:"parent_task_id"`
 	Name         string       `yaml:"name"          json:"name"`
-	Description string       `yaml:"description" json:"description"`
-	Agent       AgentConfig `yaml:"agent"     json:"agent"`
-	Timeout     Duration     `yaml:"timeout"    json:"timeout"`
-	Retry       RetryConfig  `yaml:"retry"      json:"retry"`
+	Description  string       `yaml:"description"    json:"description"`
+	Agent        AgentConfig  `yaml:"agent"          json:"agent"`
+	Timeout      Duration     `yaml:"timeout"        json:"timeout"`
+	Retry        RetryConfig  `yaml:"retry"          json:"retry"`
 	Priority    Priority     `yaml:"priority"   json:"priority"`
 	Tags        []string     `yaml:"tags"       json:"tags"`
 	DependsOn   []string     `yaml:"depends_on" json:"depends_on"`
@@ -93,20 +93,25 @@ type BatchFile struct {
 	Tasks []Task `yaml:"tasks"`
 }
 
+// validTransitions maps each state to the set of states it may transition into.
+// Terminal state COMPLETED has no outgoing edges.
+// CANCELLED, FAILED, TIMED_OUT, and BUDGET_EXCEEDED all allow re-entry at QUEUED
+// (restart or retry).
+var validTransitions = map[State][]State{
+	StatePending:        {StateQueued, StateCancelled},
+	StateQueued:         {StateRunning, StateCancelled},
+	StateRunning:        {StateReady, StateCompleted, StateFailed, StateTimedOut, StateCancelled, StateBudgetExceeded, StateBlocked},
+	StateReady:          {StateCompleted, StatePending},
+	StateFailed:         {StateQueued}, // retry
+	StateTimedOut:       {StateQueued}, // retry or resume
+	StateCancelled:      {StateQueued}, // restart
+	StateBudgetExceeded: {StateQueued}, // retry
+	StateBlocked:        {StateQueued, StateReady},
+}
+
 // ValidTransition returns true if moving from the current state to next is allowed.
 func ValidTransition(from, to State) bool {
-	transitions := map[State][]State{
-		StatePending:   {StateQueued, StateCancelled},
-		StateQueued:    {StateRunning, StateCancelled},
-		StateRunning:   {StateReady, StateCompleted, StateFailed, StateTimedOut, StateCancelled, StateBudgetExceeded, StateBlocked},
-		StateReady:     {StateCompleted, StatePending},
-		StateFailed:    {StateQueued}, // retry
-		StateTimedOut:       {StateQueued}, // retry
-		StateCancelled:      {StateQueued}, // restart
-		StateBudgetExceeded: {StateQueued}, // retry
-		StateBlocked:        {StateQueued, StateReady}, // answer received → re-queue as resume execution
-	}
-	for _, allowed := range transitions[from] {
+	for _, allowed := range validTransitions[from] {
 		if allowed == to {
 			return true
 		}
diff --git a/internal/task/task_test.go b/internal/task/task_test.go
index 637baf5..9873084 100644
--- a/internal/task/task_test.go
+++ b/internal/task/task_test.go
@@ -26,6 +26,7 @@ func TestValidTransition_AllowedTransitions(t *testing.T) {
 		{"blocked to queued (answer resume)", StateBlocked, StateQueued},
 		{"blocked to ready (parent unblocked by subtasks)", StateBlocked, StateReady},
 		{"budget exceeded to queued (retry)", StateBudgetExceeded, StateQueued},
+		{"cancelled to queued (restart)", StateCancelled, StateQueued},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {