feat(editor, inquire, cli, config): Add inline reply widget and unified editor backend

[JeanMertz]

JP now accepts replies inline — without requiring a configured external editor. Choosing r while streaming or s while tools run opens a rich, multi-line prompt built on reedline instead of spawning an editor. Ctrl+X inside the widget escapes to the external editor and re-seeds the inline buffer with the result; Ctrl+C cancels and returns to the interrupt menu.

The same inline widget drives the tool argument-edit (e), result-edit (e in the delivery confirmation), and skip-reasoning (r) flows in ToolPrompter. These options are now always offered regardless of whether editor.cmd is configured; the editor is the escape hatch, not the only path.

Two new config keys control the new behaviour:

[editor.inline]
edit_mode = "emacs"  # or "vi"

[interrupt.streaming]
reply_in_editor = false  # open editor directly, skipping the widget

[interrupt.tool_call]
reply_in_editor = false

The --edit flag on jp query is simplified to a plain boolean (--edit / --no-edit / -E); the previous --edit=<cmd> and --edit=false forms are removed.

The open-editor crate is replaced by a new EditorBackend trait in jp_editor with two methods: edit_text (ephemeral string-in/out via a temp file) and edit_file (open the editor on caller-owned paths). EditOutcome replaces the old success/error duality: a non-zero exit now maps to EditOutcome::Cancelled rather than an error, letting callers handle an aborted editor gracefully instead of propagating a failure. EditorConfig::path() is removed; all call sites use build_editor_backend().

The PromptBackend trait's text_input method is replaced by inline_reply, which returns a ReplyOutcome (Submit, Cancelled, or OpenEditor { current_text }). MockPromptBackend gains with_reply_outcomes to script full reply flows — including editor escapes — in tests.

Printer::owned_prompt_writer is added so components that need to own their output stream (the line editor) can get a Box<dyn Write + Send> that still routes through the printer's serialized worker.

This implements RFD 088.