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/transcript-cache.js

@@ -15,6 +15,7 @@ const {
 } = require("./token-usage");
 
 const MAX_CACHE_ENTRIES = 200;
+const MEMBER_TRANSCRIPT_MAX = 256 * 1024 * 1024; // S3: bytes read per pass of a member's transcript
 
 /** `YYYY-MM-DDTHH` (UTC) of a transcript timestamp, or null when it has none. */
 function utcHour(ts) {
@@ -295,10 +296,18 @@ class TranscriptCache {
     let fd;
     try {
       try {
-        fd = fs.openSync(filePath, "r");
+        fd = require("./run-as").openSafe(filePath);
       } catch {
         return this._finalizeState(state);
       }
+      // A member's transcript is read on this thread only up to a cap: a
+      // sparse file the size of a disk costs them nothing.
+      if (
+        endOffset - startOffset > MEMBER_TRANSCRIPT_MAX &&
+        require("./run-as").isMemberFile(filePath)
+      ) {
+        return this._finalizeState(state);
+      }
 
       while (pos < endOffset) {
         const want = Math.min(CHUNK, endOffset - pos);