refactor(claude-md): compose per-group CLAUDE.md from shared base + fragments

Replace the per-group "written once at init, owned by the group" CLAUDE.md
with a host-regenerated entry point that imports:

  - a shared base (`container/CLAUDE.md` mounted RO at `/app/CLAUDE.md`)
  - optional per-skill fragments (skills that ship `instructions.md`)
  - optional per-MCP-server fragments (inline `instructions` field in
    `container.json`)
  - per-group agent memory (`CLAUDE.local.md`, auto-loaded by Claude Code)

Principle: RW = per-group memory, RO = shared content. Source/skills/base
are shared; personality, config, working files, and Claude state stay
per-group.

Key changes:

  - New `src/claude-md-compose.ts` — per-spawn composition +
    `migrateGroupsToClaudeLocal()` one-time cutover.
  - New `container/CLAUDE.md` — shared base, seeded verbatim from the
    former `groups/global/CLAUDE.md`.
  - `src/container-runner.ts` — swap `/workspace/global` mount for RO
    `/app/CLAUDE.md`; call `composeGroupClaudeMd()` after
    `initGroupFilesystem()`.
  - `src/group-init.ts` — drop `.claude-global.md` symlink + initial
    `CLAUDE.md` write; seed `CLAUDE.local.md` from `opts.instructions`.
  - `src/index.ts` — call `migrateGroupsToClaudeLocal()` at startup.
  - `src/container-config.ts` — add optional `instructions` field to
    `McpServerConfig` (inline per-MCP guidance fragment).
  - `container/Dockerfile` — drop dead `/workspace/global` mkdir.
  - Remove obsolete `scripts/migrate-group-claude-md.ts`.

Migration (runs once at host startup, idempotent):

  - Delete `.claude-global.md` symlinks in each group.
  - Rename each `groups/<folder>/CLAUDE.md` → `CLAUDE.local.md`
    (preserves existing per-group content as memory).
  - Delete `groups/global/` directory.

Design docs: `docs/claude-md-composition.md` and `docs/shared-source.md`
(the latter is the sibling design discussion this refactor builds on).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/agent-runner/src/index.ts

@@ -45,6 +45,11 @@ async function main(): Promise<void> {
 
   log(`Starting v2 agent-runner (provider: ${providerName})`);
 
+  // Destinations addendum is the only runtime-generated context we inject.
+  // Agent instructions are loaded by Claude Code from /workspace/agent/CLAUDE.md
+  // (host-composed at spawn, imports /app/CLAUDE.md and fragments) plus
+  // /workspace/agent/CLAUDE.local.md (agent memory) — no need to read them
+  // manually.
   const instructions = buildSystemPromptAddendum();
 
   // Discover additional directories mounted at /workspace/extra/*