Require -- before a remote command in ssh/exec and deprecate the positional heuristic

[Vonng]

Why

Since 0.1.1, farrow ssh metaa -- uptime is refused because the token before -- must be a known node. Without --, the first token is still treated as a node only when it names one, so farrow ssh metaa uptime runs metaa uptime on the control node. That is harmless in practice (command not found) but the grammar stays ambiguous, and the help text has to explain two rules.

Proposed shape

• 0.1.x: when there is no -- and the first token is not a node, print once warning: treating "metaa" as a remote command; write farrow ssh [node] -- command.
• 0.2: the grammar becomes farrow ssh [node] [-- command...] and farrow exec [node] -- command...; a non-node token before any command is a usage error (exit 2).

Acceptance

• farrow ssh uptime warns in 0.1.x and exits 2 in 0.2; Use lines, README, and the site examples are updated together.

[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.