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/README.md

@@ -762,8 +762,8 @@ HTTP surface for the tmux-backed terminal run: one session per workspace, named
 | `GET`    | `/api/run/binary[?tool=]`     | Probe whether the CLI is on `PATH`. `?tool=codex` probes Codex; omitted or `?tool=claude` probes Claude; anything else is `400 EBADTOOL`. Returns `{ found, path }` |
 | `GET`    | `/api/run/tmux`               | `{ available }` — whether the `tmux` binary exists |
 | `GET`    | `/api/run/cwds`               | Suggested cwds (dashboard, home, recent from sessions) |
-| `POST`   | `/api/run/drop-image`          | An image dropped on the Workspace terminal. Raw body, `Content-Type` one of `image/png\|jpeg\|gif\|webp`, ≤20 MB. Saved `0600` under `<tmpdir>/agenthub-drops-<uid>/` with a RANDOM name (nothing from the request names the file); files older than a day are pruned on each write. Answers `{ path }`, which `TerminalView` pastes into the pane — what a local terminal does on a drop, and how `claude` attaches the image. 400 for any other type |
-| `POST`   | `/api/run/drop-file?name=…`    | Any file (or image) dropped on the Workspace terminal or pasted/dropped into a prompt box. Raw body of any type, ≤20 MB, same directory, `0600` and day-old pruning as `drop-image`. The name is the file's own, reduced to a `[A-Za-z0-9._-]` basename behind a random 16-hex prefix (so it can neither leave the directory nor collide, and space-joined paths stay parseable). Answers `{ path }`; 400 for an empty body or a name that sanitizes to nothing. The client uses this for every drop; `drop-image` is kept for API callers |
+| `POST`   | `/api/run/drop-image`          | An image dropped on the Workspace terminal. Raw body, `Content-Type` one of `image/png\|jpeg\|gif\|webp`, ≤20 MB. Saved `0600` under `<data dir>/drops/` (`0700`, never a guessable name in the shared tmp; S3: the member's own `~/.agenthub/handoff/drops/`) with a RANDOM name (nothing from the request names the file); files older than a day are pruned on each write. Answers `{ path }`, which `TerminalView` pastes into the pane — what a local terminal does on a drop, and how `claude` attaches the image. 400 for any other type |
+| `POST`   | `/api/run/drop-file?name=…`    | Any file (or image) dropped on the Workspace terminal or pasted/dropped into a prompt box. Raw body of any type, ≤20 MB, same directory (S3: `?laneId=` saves it AS the account that workspace's panes run as, so the pane can read it), `0600` and day-old pruning as `drop-image`. The name is the file's own, reduced to a `[A-Za-z0-9._-]` basename behind a random 16-hex prefix (so it can neither leave the directory nor collide, and space-joined paths stay parseable). Answers `{ path }`; 400 for an empty body or a name that sanitizes to nothing. The client uses this for every drop; `drop-image` is kept for API callers |
 | `GET`    | `/api/run/files?cwd=…&q=…`    | Fuzzy file search inside `cwd` for the run panel's `@`-file autocomplete. Skips `node_modules`, `.git`, `dist`, `build`, `.next`, `.cache`, `coverage`, `vendor`, etc. Cwd is required and must exist; results are capped and ranked by basename match |
 | `POST`   | `/api/run`                    | Create-or-adopt the workspace's tmux session. Body: `{ laneId, cwd, initialPrompt?, model?, permissionMode?, effort?, resumeSessionId? }`. The CLI comes from the workspace row, never from the body. An existing session whose pane sits at a shell prompt gets the argv typed into it; a pane already running a program is adopted unchanged |
 | `GET`    | `/api/run/:id`                | One run handle, or 404 once its tmux session is gone |