an unrecognised command holds the exec session open for stdin

[lex00]

Closes #18. The narrow option from that thread, approved: known verbs are untouched, only unrecognised commands hold open.

Why

The interpreter answers a known verb and is done — echo, cat, rm have said everything they are going to say. An unknown command is a different situation: a real sprite would have started a process the caller can now write to, and callers do. fountain opens exec for its runtime command and then writes the prompt in as stdin, so exiting immediately meant that write landed on a process already gone:

** (stop) exited in: GenServer.call(#PID<...>, {:write_stdin, "say hello"}, 5000)
    ** (EXIT) normal      ... ConversationServer.kick_turn/4

Its conversation server crashed on a call to a dead pid rather than getting an error back.

What changed

An unrecognised command stays open and echoes stdin back on stdout until StreamStdinEOF or close. Still not execution — the same echo the interpreter already does, extended over time.

ExecResult.Unrecognised is set by the interpreter's default branch, so the server can tell the two situations apart without guessing at the command string.

Blast radius: none for existing clients

Known verbs exit immediately exactly as before, which is what chant's Fly activities depend on. TestKnownVerbStillExitsImmediately asserts that separately from the new path so the two cannot drift.

Verified against real fountain

On k3d, with this build swapped in. Before, every turn ended:

reattach interrupted  {"reason":"no_active_session","outcome":"turn_orphaned"}

Now:

stage turn started
output  claude --dangerously-skip-permissions --print --verbose --output-format stream-json ...
output  say hello
stage turn done  {"exit_code":0}

The reply is the prompt echoed back — what an emulator should produce, and what the downstream docs already say a local conversation proves. It is the difference between fountain-ops provisioning a sandbox and fountain-ops holding a conversation offline.

go test ./... and go vet ./... green.