feat(server-s3): one Linux user per account (S3) — experimental

Server mode can now run each account as its own Linux user (ah-<name>)
when `agenthub server install-system` has set S3 up. The dashboard
reaches a member only through sudo -u / write-as / pane-launch, reads a
member's files only through owner- and type-checked opens, and hooks
are authenticated by peer uid. Local mode and server mode without S3
are unchanged.

EXPERIMENTAL: tested with fake sudo and fake accounts only. It has not
yet been verified on a real install with real member accounts, sudo or
a real Codex login. Do not offer it to real users yet.

- S3 core: provisioning, per-member tmux servers, the proxy key
  gateway, shared tools in /opt, dropped capabilities for children
  (P0–P5)
- P4: plan and quota per member home
- P6: ~/.claude.json seed; opt-in plugin baseline installed as the
  member; managed-settings spike script (not run)
- P7: Codex account pool (`agenthub accounts add --tool codex`)
- Roles merged with main's admin / analyst / developer: analysts write
  only their own rows, under S3 as well
- Per-account prefs: one implementation (Chat UI + onboarding)
- Security reviews F1–F49 (five rounds), each fix with a regression
  test: symlink/TOCTOU-safe member reads, no host writes in a member
  home, credentials and the data dir never served, owner and folder
  account must agree, pane tokens bound to the pane's uid, bounded
  sudo/tmux/git calls and member reads (no hang or crash from a member),
  async route rejections answer 500 instead of exiting
- Docs: docs/SERVER-MODE.md (S3 model, accepted ceilings, operator
  runbook), docs/plans/server-mode/S3-PLAN.md, CLAUDE.md S3 rules

Tests: server 2838/2841 (3 need 127.0.0.1:3000, held on this box);
client 1162/1162; tsc clean; headers check passes.

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

server/lib/lane-file-write.js

@@ -52,7 +52,7 @@ async function writeLaneFile(dir, requested, content, baseSha) {
     const stat = await fs.promises.stat(abs);
     if (!stat.isFile()) throw bad("ENOFILE", "no such file in the working tree");
     if (stat.size > MAX_FILE_VIEW_BYTES) throw bad("ETOOBIG", "file is too large to edit here");
-    current = await fs.promises.readFile(abs);
+    current = await require("./run-as").readFileSafeAsync(abs);
   } catch (err) {
     if (err.code === "ETOOBIG" || err.code === "ENOFILE") throw err;
     throw bad("ENOFILE", "no such file in the working tree");
@@ -70,7 +70,8 @@ async function writeLaneFile(dir, requested, content, baseSha) {
   if (eol === "\r\n") out = out.replace(/\n/g, "\r\n");
   if (text.endsWith("\n") && out && !out.endsWith(eol)) out += eol;
 
-  await fs.promises.writeFile(abs, out);
+  // As the folder's member under S3 (the dashboard reads, never writes, there).
+  require("./run-as").writeFileAs(abs, out);
   return { path: file };
 }