nssh: add Eternal Terminal (et) as a third transport

[abizer]

What

Wires Eternal Terminal (et) in as a third interactive transport alongside ssh and mosh.

• New --et flag, mutually exclusive with --ssh/--mosh.
• Auto-selection (when unforced): et → mosh → ssh. et wins when et is on the local PATH and etserver is on the remote PATH — binary-presence check only, mirroring the existing mosh-server probe.
• et launches as et <host> (resolves the host via ~/.ssh/config, default port); like the mosh path, extra ssh flags aren't forwarded. No LC_ALL override (mosh-specific).

How

• The selection decision is factored into a pure pickTransport(force, localMosh, remoteMosh, localET, remoteET) and covered by a 12-case truth-table test (transport_test.go). selectTransport feeds it real LookPath + remote command -v probes, folded into one remoteHasCommand helper. Remote SSH probes are skipped when a transport is forced or its local binary is absent.
• Since a bool can't express three transports, the session-end log's Mosh *bool becomes Transport string (ssh/mosh/et). status rendering and docs/protocol.md updated to match; old logs' mosh field is harmlessly ignored.

Why this is cleanly scoped

nssh's clipboard/URL bridge runs over ntfy and is transport-agnostic — the interactive transport is just the subprocess runSession execs after the remote-prepare step. ET slots in as a peer of mosh with no changes to the ntfy subscriber, wire format, shims, remote prepare, or collision logic.

Out of scope

• et_port config knob (default port only).
• nssh sweep for et/etterminal (etserver is a shared daemon).
• Forwarding extra ssh flags to et (mirrors the existing mosh limitation).

Verification

go build, go vet, full go test ./... green; gofmt clean. Confirmed against the built binary: usage shows [--ssh|--mosh|--et], and --ssh --et errors with the mutual-exclusion message.

Design doc: docs/superpowers/specs/2026-06-03-eternal-terminal-transport-design.md

🤖 Generated with Claude Code

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Note

Low Risk
Additive transport and logging only; ntfy bridge, shims, and auth boundaries are untouched, with unit tests on selection logic.

Overview
Adds Eternal Terminal (et) as a third interactive shell transport next to ssh and mosh, with a new --et flag (mutually exclusive with --ssh / --mosh). When no transport is forced, auto-selection is now et → mosh → ssh, using local LookPath plus remote command -v for etserver / mosh-server via a shared remoteHasCommand helper; the pure decision is in pickTransport, covered by transport_test.go.

Session logging replaces Mosh *bool on session-end with Transport (ssh / mosh / et); nssh status --tail and docs/protocol.md match. Docs (README, CLAUDE.md, internals) describe ET behavior and note the ntfy clipboard/URL bridge is unchanged.

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[chatgpt-codex-connector]

Report missing ET client instead of succeeding

When users force the new --et path on a machine without the local ET client, this branch still returns a command that cannot be started. runSession returns that start error, but nsshMain only converts *exec.ExitError into a non-zero exitCode, so this scenario prints using et, logs transport=et with exit=0, and exits successfully without explaining that et was not found. Please either check exec.LookPath("et") before selecting this forced transport or make start errors propagate as a non-zero failure.

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