feat(agents): Draw the run canvas as a real dispatch tree

.specs/features/worker-lineage/spec.md

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+# Worker lineage
+
+## Goal
+
+Show who dispatched whom. Today every session of a run shares one `runId`
+(the `open` session's id), and `codedeck run` inside a worker inherits that
+same `CODEDECK_RUN_ID`. The run canvas therefore draws every worker as a
+direct child of a card labelled "Orchestrator", even when the root is a
+`general` session and the reviewer was started by a worker, not by the root.
+
+## Verified ground
+
+- `SessionRuntime.spawn` already puts `CODEDECK_SESSION_ID=<worker id>` in
+  every worker harness environment (`src/drivers/session-runtime.ts`).
+- `open` puts `CODEDECK_RUN_ID=<open id>` in the harness environment; the open
+  session row has `origin: "open"` and `runId === id`.
+- `session.list` returns full session rows, so new columns reach
+  `codedeck ps --json` and the agents pane without extra plumbing.
+
+## Acceptance criteria
+
+- AC1: A session row SHALL carry `parentId`, the session that dispatched it,
+  and `role`, the CodeDeck role it was started with.
+- AC2: `codedeck run` SHALL send `parentId = CODEDECK_SESSION_ID`, falling
+  back to `CODEDECK_RUN_ID`, and `role = parseRole(--role)`.
+- AC3: `open` SHALL set `CODEDECK_SESSION_ID` to its own id and send its role
+  on `session.adopt`.
+- AC4: The daemon SHALL record `parentId` only when the parent exists in the
+  store, and SHALL inherit the parent's `runId` when the request has none.
+- AC5: The run canvas SHALL draw a tree: each card hangs from its dispatcher.
+  A parent outside the snapshot, or a legacy row with none, hangs from the root.
+  A parent loop is cut at the root.
+- AC6: The root card SHALL be labelled with the open session's role, falling
+  back to "Orchestrator"; worker cards SHALL show their role.
+- AC7: A finished worker SHALL stay drawn as a card while any descendant is
+  live; history lines SHALL name the dispatcher when it is not the root.
+- AC8: When the pane is short, a card SHALL be evicted only after all its
+  children, so the drawn tree stays connected.
+
+## Out of scope
+
+- Moving reviewer dispatch from the worker prompt into the harness (CodeDeck
+  starting the reviewer when a general finishes). The lineage edge written
+  here is what that later change will fill from the daemon side.
+- The web console canvas.