[travsr-cli] Fix #777: refuse travsr mcp on a terminal instead of hanging

[ritikpal1122]

Closes #777.

travsr mcp speaks JSON-RPC over stdin, so run directly in a terminal it blocked on a request that is never typed: no output, no prompt, no indication anything was wrong. The report describes a process that "appears frozen".

$ travsr mcp
error: travsr mcp speaks JSON-RPC over stdin and is meant to be launched by an
MCP client, not run directly.
stdin is a terminal here, so there is no client to talk to and the server would
wait forever for a request that never arrives.

To connect it to your editor or agent: travsr connect
To drive the protocol by hand: TRAVSR_MCP_ALLOW_TTY=1 travsr mcp

Two decisions worth calling out

Guarded on stdin, not stdout. A client that pipes stdin while leaving stderr on the terminal is a normal working setup, so testing stdout would refuse a legitimate launch. stdin being a terminal is the condition that actually makes the server unusable, because there is no client on the other end to send a request.

TRAVSR_MCP_ALLOW_TTY keeps the escape hatch. Pasting a frame by hand to probe a response is a real debugging workflow, and a guard with no override would remove it.

Verified against the built binary, not only the unit test

All three paths, because the failure mode here is behavioural:

TTY               -> refuses and exits (was: hangs forever)
piped stdin       -> {"id":1,...,"serverInfo":{"name":"travsr","version":"1.0.0"}}
ALLOW_TTY=1 + TTY -> serves, no refusal

The middle one matters most: it is the regression this guard could plausibly cause, and it does not.

On the test

It asserts the wording, not the TTY branch. Faking a terminal on stdin is not portable, and the branch itself is a single is_terminal call. What can regress silently is the message, and the entire point of the issue is that the user was left with nothing to act on, so a message that stopped naming travsr connect or the override would reintroduce the dead end in a quieter form.

Verification

cargo test -p travsr-cli --bin travsr                     252 passed, 0 failed
cargo clippy -p travsr-cli --all-targets -- -D warnings   clean
cargo fmt --all -- --check                                clean
check-em-dash.sh                                          OK

One file changed.