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/db.js

@@ -1768,9 +1768,9 @@ db.exec(`
     created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
     expires_at TEXT NOT NULL
   );
-  -- One account's own UI preferences (server/routes/me.js). \`local\` in local
-  -- mode, \`u<id>\` per server-mode account. The same table as the server-s3
-  -- work's (which adds theme, language… to PREF_KEYS) — keep the two identical.
+  -- One account's own UI preferences (theme, language, pet, …) and whether it
+  -- went through the first-run setup (server/routes/me.js). \`local\` in local
+  -- mode, \`u<id>\` per server-mode account.
   CREATE TABLE IF NOT EXISTS user_prefs (
     user_key TEXT PRIMARY KEY,
     prefs TEXT NOT NULL DEFAULT '{}',