feat(tasks): task_decompose splits a goal into a dependency-linked graph in one approval

The freestyle analyst can now break a large goal with dependency seams or
independent review/handoff points into a parent epic + DEPENDS_ON-linked children
in a single T2 approval, instead of N separate task_create calls. Parent
DEPENDS_ON every child (completes last); each child IMPLEMENTS parent. Resolves
depends_on by ref or index; rejects cycles, unresolved refs, missing title/goal,
and empty batches; same batch dedup + force_reason convention as task_create.

A session works one active task, so multi-task work is multi-session by
construction: the analyst names the single ready task this run works, the architect
threads only that one, and siblings are claimed by later runs via task_ready
(claim-driven; no scheduler, /tasks/next stays rejected).

Doctrine: analyst_freestyle.md picks one-task-vs-decompose and names the ready
task; architect_freestyle.md threads only that one; plus the L0 policy line.
freestyle_planning.toml analyst gains task_decompose (pinned by
FreestylePlanningWorkflowTest).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infrastructure/workflow/src/test/kotlin/com/correx/infrastructure/workflow/FreestylePlanningWorkflowTest.kt

@@ -70,9 +70,10 @@ class FreestylePlanningWorkflowTest {
 
         assertEquals(2, graph.transitions.size)
 
-        // analyst frames the work: search + open a task (the architect threads it into the plan).
+        // analyst frames the work: search + open one task or decompose into a graph (the architect
+        // threads the named task into the plan).
         assertEquals(
-            setOf("file_read", "ShellTool", "task_search", "task_context", "task_create"),
+            setOf("file_read", "ShellTool", "task_search", "task_context", "task_create", "task_decompose"),
             graph.stages[StageId("analyst")]!!.allowedTools,
         )
     }