feat(input): add multiline support

[kunchenguid]

Summary

Adds multiline input editing to the gsh REPL. When the user presses Enter on incomplete input (unclosed quotes, heredocs, trailing pipes, control structures), a newline is inserted and a continuation prompt is shown instead of submitting. Alt+Enter force-inserts a newline regardless of completeness. A new gsh.continuationPrompt SDK property allows customization, with Starship integration out of the box.

Risk Assessment: 🟢 Low — Well-tested multiline input feature scoped to the REPL input subsystem with minor critique warnings and no test failures.

Architecture

flowchart TD
    keymap["KeyMap / ActionInsertNewline (updated)"]
    handlers["handlers.go (updated)"]
    multiline["IsInputComplete (added)"]
    input_model["input.Model (updated)"]
    renderer["Renderer (updated)"]
    highlight["renderStyled / splitHighlightedByNewlines (added)"]
    prediction["onTextChanged (updated)"]
    repl["REPL.Run / getContinuationPrompt (updated)"]
    sdk["gsh.continuationPrompt (added)"]
    repl_context["REPLContext.ContinuationPromptValue (added)"]
    starship["starship.gsh (updated)"]
    docs["gsh-object docs (updated)"]
    sh_parser["mvdan/sh syntax.Parser (unchanged)"]

    subgraph input_pkg["internal/repl/input"]
        direction TB
        keymap -->|"Alt+Enter maps to"| input_model
        input_model -->|"Enter delegates to"| handlers
        handlers -->|"checks completeness via"| multiline
        multiline -->|"parses with"| sh_parser
        handlers -->|"inserts newline into"| input_model
        input_model -->|"renders via"| renderer
        renderer -->|"splits highlighted text via"| highlight
        input_model -->|"disables predictions via"| prediction
    end

    subgraph repl_pkg["internal/repl"]
        direction TB
        repl -->|"reads continuation prompt from"| repl_context
        repl -->|"passes ContinuationPrompt to"| input_model
    end

    subgraph sdk_pkg["internal/script/interpreter"]
        direction TB
        sdk -->|"backed by"| repl_context
    end

    subgraph defaults_pkg["cmd/gsh/defaults"]
        direction TB
        starship -->|"sets"| sdk
    end

    docs -.->|"documents"| sdk

Loading

Key changes made

• Incomplete input detection — New IsInputComplete() in multiline.go uses mvdan/sh parser to distinguish incomplete input (needs more lines) from complete or errored input (should submit).
• Enter key behavior — handleSubmit() now checks completeness before submitting; incomplete input gets a newline inserted. New handleInsertNewline() (Alt+Enter) always inserts a newline.
• Multi-line rendering — render.go gains renderMultiLine() which renders each line with the correct prompt (main or continuation) and preserves cross-line syntax highlighting via splitHighlightedByNewlines() with ANSI state tracking. Predictions are disabled for multi-line input.
• gsh.continuationPrompt SDK property — New read/write string property on the gsh object backed by REPLContext.ContinuationPromptValue. Defaults to "> ".
• Starship integration — starship.gsh caches starship prompt --continuation at init and applies it via gsh.continuationPrompt.
• Input sanitization — sanitizeRunes() now preserves \n instead of replacing it with a space, and normalizes \r\n/\r to \n.
• Renderer refactoring — Existing appendTextWithWrapping and appendTextWithWrappingAndCursor are refactored into appendPreHighlightedWithWrapping and appendPreHighlightedWithCursor variants that accept pre-highlighted text, with a shared consumeANSISequence() helper to eliminate duplication.
• Tests — 346-line multiline_test.go covers completeness detection, submission behavior, continuation prompts, multi-line rendering, cross-line syntax highlighting, and key bindings.

How was this tested

• All 24 test packages pass
• 346 lines of new tests in multiline_test.go covering completeness detection, submission behavior, continuation prompts, multi-line rendering, cross-line syntax highlighting, and key bindings

[kunchenguid]

Critique review comments