fix(repl): exit on Ctrl-C at an empty prompt, discard the continuation otherwise

[devin-ai-integration]

What and why

Ctrl-C in the sysml REPL never ended a session: the readline adapter turned readline.ErrInterrupt into an empty line. That also meant Ctrl-C at a ...> continuation prompt acted like a blank line and submitted the half-typed buffer.

Now the adapter reports the interrupt and repl.Loop decides what to do:

// cmd/sysml/prompt_readline.go
if err == readline.ErrInterrupt { return line, repl.ErrInterrupt }

// internal/frontend/repl/loop.go
if errors.Is(err, ErrInterrupt) {
    if buf.Len() == 0 && line == "" { return nil }   // empty sysml> prompt: exit, same as %quit / Ctrl-D
    buf.Reset(); prompt = primaryPrompt; continue    // otherwise discard line + continuation
}

So Ctrl-C with text on the line clears it, a second Ctrl-C quits (similar to the Node REPL). At a continuation prompt, Ctrl-C now drops the buffer without parsing it. A blank line still submits the buffer.

The wasm plainReader never returns an interrupt, so it isn't affected. Ctrl-C while a command is running is unchanged: readline is out of raw mode by then, and the default SIGINT already ended the process.

CI: timeout-minutes for the static-and-integrity job in .github/workflows/pr.yml goes from 20 to 30. make lint alone takes about 12 of the 20 minutes. The job was cancelled at the limit on this PR (twice) and on #800, both times after the REPL no-solver gate had passed and before the last corpus gates ran.

How it was verified

• New TestLoopInterrupt in internal/frontend/repl covers each case: a typed line is discarded, a buffered continuation is discarded (never submitted), the next submission still works, and Ctrl-C at an empty primary prompt ends the loop before later lines are read.
• Ran the real binary under a pty with expect, then again in a recorded GUI terminal (see the PR comment). Ctrl-C discarded both a typed line and a ...> continuation, and %eval 1 + 1 still gave = 2. Ctrl-C at the empty prompt exited with status 0, both in a plain session and after loading a file. Ctrl-D, %quit, a blank-line submit, and cancelling tab completion or reverse search are unchanged.
• go test ./internal/frontend/repl ./cmd/sysml: passes, apart from the OPENSYSML_REQUIRE_*_CORPORA tests, which fail only because the corpora are missing on this machine. go vet, gofmt and staticcheck are clean on the changed packages, and scripts/changelog.py check passes.
• Merged develop after fix(migrate): pass the uniqueness argument the read result pin's shaped multiplicity takes #800 landed, which fixes the shaped compile error that broke the first CI run.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md): Ctrl-C row in docs/reference/repl-commands.md; the REPL testing skill's continuation note
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (no gate count moved)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

Link to Devin session: https://nasa-jpl-demo.devinenterprise.com/sessions/160aea0b6827433b98692b4c13892285
Open in Devin Desktop: https://nasa-jpl-demo.devinenterprise.com/desktop/session/160aea0b6827433b98692b4c13892285?variant=devin
Requested by: @HuiJun

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

Tested in a real terminal (Konsole) on 387663e83

• Ctrl-C threw away a typed line and both an empty and a non-empty ...> continuation. Nothing was submitted and no diagnostics printed. Afterwards %eval 1 + 1 returned 2, and %list did not show the discarded packages.
• Ctrl-C at an empty sysml> prompt exited with status 0, both in a plain session and after loading a model file.
• Regressions: Ctrl-D and %quit still exit 0. A blank line at ...> still submits the buffer. Ctrl-C closes the tab-completion menu and cancels reverse search without leaving the REPL.

Discard and exit	Exit after loading a file

The test binary included a local fix for the shaped compile error on develop (the one #800/#801 fix). That fix is not part of this PR.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.