OXY-162: New Oxygen command-execution API (CommandService / Command2)

[Kalin-Rudnicki]

Derives from the current/feature/command-improvements brainstorm branch (a chain of
"WIP : savepoint" commits) and brings that exploratory work to a compiling, tested state.

Jira: OXY-162

What the new command API is

A "v2" API in oxygen-zio for running external OS processes from ZIO, intended to eventually
replace the legacy oxygen.zio.system.Command (which shells out via scala.sys.process):

• CommandService — pluggable execution SPI (executeSync, executeSyncStreamErr,
  executeCode + derived helpers), held in a FiberRef with a platform-specific default.
• Command2 — immutable fluent builder (command / args / cwd / env / sudo) building a
  BuiltCommand; execute surface for sync capture, streamed stderr, exit-code, and typed
  decode-to-A (String / PlainText schema / JSON), plus legacy-compatible execute* shims.
• JavaCommandService — JVM/Native impl over java.lang.ProcessBuilder: scoped process
  lifecycle, concurrent stdout/stderr draining (avoids pipe-buffer deadlock), stdin sources.
• UnimplementedCommandService — Scala.js default (typed Unimplemented failure).
• stdio ADTs (CommandInputSource / CommandOutputSource / ShowCommand) and a CommandError ADT.
• Supporting: StringDecoder.DecodingFailure and json.JsonError now extend oxygen Error.

What this PR did to finish it

• Fixed a blocking bug: CommandInputSource.Empty mapped to Redirect.DISCARD (a WRITE-only
  redirect), so the process failed to start for every no-stdin execution — i.e. the API failed on
  first use. Now uses PIPE + closes the child's stdin (EOF).
• Restored JS/Native compilation: the shared JavaCommandService used ZSink.fromOutputStream
  (JVM-only) and ProcessBuilder (absent on JS). Rewrote the stream-stdin path to a portable
  blocking chunk-write (Native compiles it), and excluded the file from the JS source set.
• Resolved WIP loose ends: corrected BuiltCommand shell-escaping to the POSIX '\'' idiom
  (display/logging only — execution never goes through a shell), documented the Command2 vs legacy
  Command coexistence, cleaned up FIX-PRE-MERGE notes.
• Added tests: Command2Spec (13 tests) — builder, escaping, and live JVM execution.

Verification

• oxygen-zio compiles on JVM / JS / Native (JS and Native were broken before).
• sbt utJVM/test → 743 passed, 0 failed (new Command2Spec + legacy CommandSpec + all
  core/schema/json/zio specs; confirms the Error-trait migration caused no regressions).
• sbt oxygen-executableJVM/Test/compile succeeds (downstream consumer of the changed core/json APIs).
• sbt fmt applied.

Honest mergeability assessment

Reviewable and non-broken, but an incremental landing of an in-progress API (~6.5/10). The JVM
happy paths are solid, test-covered, and green, and the previously API-breaking Empty-stdin bug is
fixed. Caveats: Command2 is still parallel to the legacy Command (no call-site migration yet);
the JS backend is intentionally unimplemented; Native compiles but is not exercised by any
test; and several stdio/decode branches (Stream/File/Log, streamed-stderr, JSON/PlainText decode)
are only lightly or not covered. Safe to merge behind the existing unused Command2 name; not a
finished, fully-migrated replacement for Command.

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