Add fold exec — run a command in a running machine

[blakebauman]

Context

Next operability gap after central logs (#2): there was no way to get into a deployed machine to inspect it — no exec, no ssh. For an agent runtime, "run a command inside my agent's machine" (ps, env, cat /data, curl localhost) is a core debugging primitive. This adds one-shot command exec.

Stacked on #2 (feat/central-machine-logs) because both extend the Runtime interface. Base will retarget to main once #2 merges; review the exec commit for this PR's changes in isolation.

What changed

• Runtime gains Exec. The docker runtime implements it via docker exec (forwards stdin, surfaces the command's exit code). wasm (no shell), microVM, and firecracker return ErrExecUnsupported — microVM exec over vsock/ssh is roadmap.
• Agent admin API serves POST /v1/machines/{id}/exec alongside the existing logs route, with a per-command timeout.
• Control resolves the target machine (the named one, or the first running one), proxies to the owning host agent, and records an exec event + audit entry.
• API: POST /v1/apps/{app}/exec, gated by the deploy scope.
• CLI: fold exec [-a app] [--machine id] -- <cmd> prints combined output, forwards piped stdin, and exits with the command's exit code.

Scope

One-shot, buffered exec bounded by --timeout (default 60s). Interactive TTY sessions and long-lived streams (tail -f) are out of scope for this first version — they need bidirectional streaming (websocket/hijack) and can follow.

Tests

• control: machine-target resolution — auto-picks a running+scheduled machine, rejects unscheduled/unknown ids, errors when none running
• api: scope + validation — viewer JWT → 403, missing cmd → 400, valid request with no running machine → 502
• agent: wasm Exec returns ErrExecUnsupported

Verification

• gofmt -l clean · go vet ./... clean · go test ./... all pass · go build -tags firecracker ./... compiles
• Manual (local docker deploy of hello-go): echo output, env/hostname (PORT=8080, m-0), stdin pipe (cat), and exit-code propagation (exit 7 → CLI exits 7) all verified

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