feat(sandbox): thin cos onto createos-cli's offload/matrix/fork --count

[pratikbin]

Summary

Sub-project C of the sandbox-surface work: thins scripts/cos onto the compositions createos-cli just shipped (createos-cli#84), and adds the matrix verb the plugin didn't have before.

• cos offload now delegates to createos sandbox offload <dir> -- <cmd> instead of a ~65-line hand-rolled stage/push/exec/keepalive/pull/destroy sequence. Same flags (minus -w, see below), same UX, less code to maintain, and it inherits the Go implementation's retry-on-connection-failure and no-billable-leak-on-teardown-failure behavior for free.
• cos fanout now delegates to createos sandbox matrix <dir> --job ... --job ... (no --prepare) — each job runs on a fork of one staged box rather than an independently staged box. Same isolation (every job started from an identical staged state either way), faster (fork is a ~1s server-side copy).
• New cos matrix / /createos-sandbox:matrix — fanout plus -P '<setup>', which runs once on the golden box before it forks. For the case two or more jobs share the same dependency install or toolchain prep.
• cos fork keeps its own pause-then-resume orchestration around the project box (legitimate — it's forking a box it owns, on request), but the fork step and id resolution now delegate to createos sandbox fork -o json, which returns the new id directly. This deletes the comm-diff-against-sandbox ls hack and the multibyte/set -u trap that came with it (see the removed comment in the old code). Gained -c N for direct N-way cloning.
• -w/--swap dropped from offload — devbox:1 can't swapon a file added post-boot from most shapes anyway, so the flag bought little for the maintenance cost. Documented as something to compose into the command string instead ('fallocate -l 4G /swapfile && mkswap /swapfile && swapon /swapfile && <cmd>').
• Docs updated throughout — skill, references, command frontmatter, README — for the new/changed verbs, and to correct a stale "2 concurrent boxes" quota claim to the actually-observed number (10).

A real bug found and fixed along the way

Live-testing this against createos-cli's feat/sandbox-compositions branch (five scenarios in tmux, drawn from the product's own use-case catalog — untrusted-code offload, independent-job fanout, shared-setup matrix, project-box fork, and a background job self-deleting via the loopback signal endpoint) caught a real bug: fork <sandbox> --count 2 silently created one clone instead of two, with no error.

Root cause: Go's stdlib flag package (under urfave/cli) stops parsing at the first non-flag argument. fork <sandbox> --count 2 — sandbox id first, the order every caller actually writes, cos included — never has --count parsed as a flag at all; it lands unread in c.Args(), and c.Int("count") silently returns the flag's default (1). The existing regression test for --count validation only ever wrote --count before the id, so it never exercised this path.

Fixed on the createos-cli side (feat/sandbox-compositions, now pushed to createos-cli#84) by reusing the raw-argv fallback already built for process run --cwd (commit 8c1f7ac) — confirmed fixed with a new regression test using the natural argument order, and reconfirmed live before writing this description.

Test plan

• bash -n + shellcheck -x clean on scripts/cos
• Live, via COS_CLI=/tmp/createos-dev against createos-cli#84: offload (locked egress, blocked reaching an unlisted host), fanout (4 independent jobs, correct mixed exit codes), matrix -P (3 jobs, prepare output identical across all forks — proves it ran once), fork -c N (2 clones from one call, after the count fix), self-signal (curl 127.0.0.1:1029/self/delete from inside a cos run job — confirmed via sandbox get showing status: destroyed)
• No sandboxes leaked across any of the above — verified against account sandbox count before/after, including cleanup of a few stragglers from debugging the count bug itself