Require an explicit -- before Barn remote commands

[Vonng]

Current behavior

barn ssh and barn exec accept a command without --. A known first argument selects a node; otherwise the command runs on the default node with a warning. Near-miss node names are rejected.

Proposed behavior

Require -- before remote commands:

• barn ssh [node] [-- command...]
• barn exec [node] -- command...

An unknown node or a command without the separator should be a usage error. Keep help, completion, examples, and CLI tests consistent with the selected grammar.

This is a proposed CLI change; it is not implemented by the naming cleanup.

[Vonng]

Plan for the 0.1.x half

This change will deliberately stop at the deprecation warning; the 0.2 grammar enforcement remains open in this issue.

• Keep splitRemoteInvocation pure: it will return whether the no---, first-token-is-not-a-node compatibility heuristic was used.
• runPrivateSSH will emit exactly one warning on stderr at the command boundary, naming the token and the unambiguous farrow ssh|exec [node] -- command spelling.
• Add table-driven parser coverage plus command-boundary warning coverage for ssh/exec, known-node, explicit---, and empty invocations.
• Record the user-visible warning in [Unreleased].
• Preserve all accepted 0.1.x invocations and exit codes. No Use line, README, or site grammar change belongs in this half; those move atomically with 0.2 enforcement.

The closing commit for this half will reference #11 but will not use Closes, and the Done comment will explicitly leave the issue open.

[Vonng]

Done for 0.1.x; issue intentionally remains open

Commit 9791288 adds the compatibility warning without changing the accepted grammar or exit status:

• The parser reports when the no---, non-node-first-token heuristic was used; it remains free of output side effects.
• ssh/exec emit exactly one command-specific warning at the command boundary.
• Empty invocations, known-node selectors, and explicit -- invocations do not warn.
• Table and command-boundary tests cover both command names and every warning/no-warning branch.
• [Unreleased] documents the deprecation behavior.
• Local make check passed.
• GitHub Actions ci run 33369830960 passed all gates.

The commit uses Refs #11, not Closes #11. The 0.2 enforcement plus simultaneous Use/README/site grammar update remains the unfinished half of this issue.

[Vonng]

Final status summary — OPEN / 0.1.x phase completed

Why this exists: without --, a first token that is not a known node is treated as a remote command. That preserves the 0.1 grammar, but a misspelled node such as metaa can therefore become a command on the control node.

Completed now: 9791288 makes the parser report when this compatibility heuristic was used and emits exactly one command-specific warning: use farrow ssh|exec [node] -- command. Known-node, empty, and explicit--- forms do not warn. Behavior and exit codes remain compatible; tests, make check, and CI 33369830960 passed.

Why it remains open: the 0.2 phase still needs to reject a non-node token before the command, require the unambiguous separator grammar, and update Use lines, README, and the public site atomically. The commit intentionally uses Refs #11, not Closes #11.