feat(server): workspace_panes envelope + read_tab_by_number tool

Widen the sessions.workspace_panes JSONB from a bare WorkspacePane[] to a
WorkspaceState envelope { panes, tabNumbers, nextTabNumber, closedPaneStack }.
The PATCH validator accepts either the legacy array or the envelope (zod union)
and normalizes to a full envelope before storing, so existing array-shaped rows
migrate transparently on next write. The session_workspace_updated WS frame
schema is widened to match (kept byte-identical to the web copy; parity test
passes).

Adds read_tab_by_number, a read-only tool that resolves a session-scoped tab
number to its chat via the persisted tabNumbers map and returns that chat's
transcript (oldest-first, sentinels skipped, capped at 20k chars). Tools gain an
optional ToolExecCtx ({ sql, sessionId }) 4th param on ToolDef.execute, threaded
through executeToolCall from executeToolPhase; the param is optional so existing
filesystem tools and the apps/coder consumer stay compatible. Registered in
ALL_TOOLS + READ_ONLY_TOOL_NAMES.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/routes/sessions.ts

@@ -28,18 +28,20 @@ const HtmlArtifactStateZ = z.object({
   title: z.string().max(500),
 });
 
+const PaneKindZ = z.enum([
+  'chat',
+  'terminal',
+  'coder',
+  'agent', // legacy alias — normalized to coder on write
+  'empty',
+  'settings',
+  'markdown_artifact',
+  'html_artifact',
+]);
+
 const WorkspacePaneZ = z.object({
   id: z.string().min(1).max(200),
-  kind: z.enum([
-    'chat',
-    'terminal',
-    'coder',
-    'agent', // legacy alias — normalized to coder on write
-    'empty',
-    'settings',
-    'markdown_artifact',
-    'html_artifact',
-  ]),
+  kind: PaneKindZ,
   chatId: z.string().min(1).max(200).optional(),
   chatIds: z.array(z.string().min(1).max(200)).max(50),
   activeChatIdx: z.number().int(),
@@ -47,8 +49,27 @@ const WorkspacePaneZ = z.object({
   html_artifact_state: HtmlArtifactStateZ.optional(),
 });
 
+// v2.6.x: workspace_panes column widened from a bare WorkspacePane[] to a
+// WorkspaceState envelope (panes + stable session-scoped tab numbering +
+// reopen stack). closedPaneStack entries are lighter than full panes — just
+// the kind + chat ids needed to recreate a closed pane on reopen.
+const ClosedPaneEntryZ = z.object({
+  kind: PaneKindZ,
+  chatIds: z.array(z.string().min(1).max(200)).max(50),
+  activeChatIdx: z.number().int(),
+});
+
+const WorkspaceStateZ = z.object({
+  panes: z.array(WorkspacePaneZ).max(10),
+  tabNumbers: z.record(z.string(), z.number().int()).default({}),
+  nextTabNumber: z.number().int().default(1),
+  closedPaneStack: z.array(ClosedPaneEntryZ).max(10).default([]),
+});
+
+// Accept either the legacy bare array OR the envelope. The handler normalizes
+// to a full envelope before storing (see MIGRATION rule in the PATCH handler).
 const WorkspacePanesBody = z.object({
-  workspace_panes: z.array(WorkspacePaneZ).max(10),
+  workspace_panes: z.union([z.array(WorkspacePaneZ).max(10), WorkspaceStateZ]),
 });
 
 const PatchBody = z.object({
@@ -308,12 +329,20 @@ export function registerSessionRoutes(
         reply.code(400);
         return { error: 'invalid body', details: parsed.error.flatten() };
       }
-      const workspacePanes = parsed.data.workspace_panes.map((pane) =>
+      // v2.6.x MIGRATION: the body is either a legacy bare WorkspacePane[] or
+      // the WorkspaceState envelope. Normalize to a full envelope so the column
+      // always stores the envelope shape going forward.
+      const body = parsed.data.workspace_panes;
+      const envelope = Array.isArray(body)
+        ? { panes: body, tabNumbers: {}, nextTabNumber: 1, closedPaneStack: [] }
+        : body;
+      // agent → coder normalization on the panes array (unchanged write rule).
+      envelope.panes = envelope.panes.map((pane) =>
         pane.kind === 'agent' ? { ...pane, kind: 'coder' as const } : pane,
       );
       const rows = await sql<Session[]>`
         UPDATE sessions
-        SET workspace_panes = ${sql.json(workspacePanes as never)},
+        SET workspace_panes = ${sql.json(envelope as never)},
             updated_at = clock_timestamp()
         WHERE id = ${req.params.id}
         RETURNING id, project_id, name, model, system_prompt, status, created_at, updated_at,