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-agents.js

@@ -1,6 +1,7 @@
 /**
  * @file Cleans up what older builds wrote into a lane: `removeStageDoc` takes
- * AgentHub's stage block back out of a codex lane's AGENTS.md. Plain file I/O.
+ * AgentHub's stage block back out of a codex lane's AGENTS.md. Plain file I/O,
+ * so never in an S3 member's folder.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -22,6 +23,10 @@ const STAGE_BLOCK_END = "<!-- agenthub:stage:end -->";
 function removeStageDoc(cwd) {
   const file = path.join(cwd, "AGENTS.md");
   try {
+    // S3: never in a member's folder — the host would read, rewrite or delete
+    // through any link the member planted there (with CAP_DAC_READ_SEARCH).
+    // Builds that wrote the block predate S3; a member removes it themselves.
+    if (require("./run-as").ownerForRead(cwd)) return false;
     const doc = fs.readFileSync(file, "utf8");
     const start = doc.indexOf(STAGE_BLOCK_START);
     const end = doc.indexOf(STAGE_BLOCK_END, start);