Run Litestream commands without a shell and surface failures

[cole-robertson]

First of two to bring the gem to Litestream 0.5. This one stands alone and fixes a bug that exists on 0.3.13 today.

Problem

Commands.run joined argv into one shell string, ignored the exit status, and discarded stderr. A failing command returned "" (or [] after table parsing) and looked like success; arguments with spaces broke; argv reached the shell unescaped.

Reproduced against 0.5.17: ltx on a database that is not in the config exits 1 with Error: database not found in config on stderr and nothing on stdout. The old wrapper returned [], indistinguishable from "nothing replicated yet". The 0.3.13 binary uses the same exit and stderr conventions.

Change

One process path: Open3.popen3(*cmd, pgroup: true), argv array, no shell.

• Non-zero exit raises CommandFailedException with the command, exit status, and stderr.
• Explicit output mode instead of guessing from argv: :table (existing parsing), :raw, or :json when a caller passes json: true (Litestream ≥ 0.5). Restore's two opt-in skips (-if-db-not-exists on an existing output, -if-replica-exists with no backups) exit 0 and print one logfmt line on stdout even with -json; those return {"skipped" => true, "message" => ...}.
• timeout: kills the process group (TERM, then KILL after a one-second grace, unconditionally) on expiry and raises CommandTimeoutException with the child reaped. The deadline also covers the pipe readers, so a descendant that outlives the direct child is caught.
• Option values are stringified before spawning; Process.spawn rejects Integers, which the old shell-string runner accepted and the README documents (--parallelism 10).
• The LITESTREAM_INSTALL_DIR note prints once per process.
• The old error-row sniffing in execute is gone; it only existed because failures were swallowed.

Public method signatures and default return shapes are unchanged. prepare still returns argv.

Verified

• 102 tests, 0 failures; standardrb clean; fork CI (Ruby workflow) green.
• New TestRunner class drives a fake executable: table, JSON object/array, empty list, both skip lines, non-zero exit with stderr in the message, an argument containing a space, timeout with reap, timeout with a descendant that ignores TERM, an Integer option value, note printed once.
• Real 0.5.17: databases in table and JSON modes, restore -json returning txid, the skip case, a failing restore raising with the stderr message, a 0.2 s timeout killing a sleeping fake with no orphan left.
• Running in production at Rebulk (Nightrail Cloud) for the readiness probe and restore drill.