refactor(v2): per-group filesystem init, persistent across spawns

Each group's on-disk state (CLAUDE.md, .claude-shared/, agent-runner-src/)
is now initialized exactly once at group creation and owned by the group
forever after. Spawn does only mounts — no copies, no settings.json
overwrites, no skill clobbers, no source resyncs.

Global memory composition switches from "host reads /workspace/global/CLAUDE.md
at bootstrap and stuffs it into systemPrompt.append" to "group CLAUDE.md
imports it via @/workspace/global/CLAUDE.md at the top." Edits to global
propagate instantly through the existing read-only mount; no copy, no
restart.

- src/group-init.ts: new initGroupFilesystem(group, opts?) — idempotent,
  populates groups/<folder>/, .claude-shared/, agent-runner-src/ only when
  paths don't already exist.
- src/container-runner.ts: buildMounts() calls init defensively at the
  top (catches existing groups on first spawn after this change), drops
  the inline settings.json write, skills cpSync loop, and agent-runner-src
  rm-then-copy. Just mounts now.
- src/delivery.ts: create_agent flow uses initGroupFilesystem with
  optional instructions, replacing the inline mkdirSync + writeFileSync.
- container/agent-runner/src/index.ts: drops GLOBAL_CLAUDE_MD reading.
  systemContext.instructions is now only the runtime-generated
  destinations addendum.
- scripts/migrate-group-claude-md.ts: one-shot migration that prepends
  the @-import to existing groups' CLAUDE.md. Skips if global doesn't
  exist or if the @-import is already present (regex match on the @ form
  to avoid false positives from prose mentions of the path).
- groups/main/CLAUDE.md: prepended by the migration.

Existing groups need a one-time wipe of their agent-runner-src/ dir so
init re-populates from current host source — done locally before this
commit. Future host-side updates to container/skills/ or
container/agent-runner/src/ won't auto-propagate; that's the trade-off
for unconditional persistence and will be covered by host-mediated
refresh tools in a follow-up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/delivery.ts

@@ -51,8 +51,9 @@ import {
   writeSystemResponse,
 } from './session-manager.js';
 import { resetContainerIdleTimer, wakeContainer } from './container-runner.js';
+import { initGroupFilesystem } from './group-init.js';
 import type { OutboundFile } from './channels/adapter.js';
-import type { Session } from './types.js';
+import type { AgentGroup, Session } from './types.js';
 
 const ACTIVE_POLL_MS = 1000;
 const SWEEP_POLL_MS = 60_000;
@@ -509,7 +510,7 @@ async function handleSystemAction(
       const agentGroupId = `ag-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`;
       const now = new Date().toISOString();
 
-      createAgentGroup({
+      const newGroup: AgentGroup = {
         id: agentGroupId,
         name,
         folder,
@@ -517,12 +518,9 @@ async function handleSystemAction(
         agent_provider: null,
         container_config: null,
         created_at: now,
-      });
-
-      fs.mkdirSync(groupPath, { recursive: true });
-      if (instructions) {
-        fs.writeFileSync(path.join(groupPath, 'CLAUDE.md'), instructions);
-      }
+      };
+      createAgentGroup(newGroup);
+      initGroupFilesystem(newGroup, { instructions: instructions ?? undefined });
 
       // Insert bidirectional destination rows (= ACL grants).
       // Creator refers to child by the name it chose; child refers to creator as "parent".