markdown rendering in TTY

[emesal]

using streamdown-rs (https://github.com/fed-stew/streamdown-rs, MIT license)

activates when both conditions are met:
a) stdout is a TTY
b) --json-output is not set

note: each tool-call round creates a fresh markdown parser, since each round is a separate LLM response stream. this means that parser state (being inside a code fence etc) does not carry between rounds.

remaining work before merge:

• add --raw CLI option to force passthrough/no markdown rendering
• add config.toml/local.toml option to enable/disable markdown renderer
• ensure adequate tests are implemented
• documentation updates
• fmt clippy etc

the deets:

new src/markdown.rs: MarkdownStream struct with

• TTY detection via std::io::IsTerminal, streamdown pipeline for terminals, passthrough for piped output
• calling chibi with --json-output forces passthrough
• line buffering in write_chunk() accumulates chunks and processes complete lines w Parser::parse_line() -> Renderer::render_event()
• finish() flushes remaining partial line and calls Parser::finalie() to close open blocks
• StdoutWriter newtype so the Renderer can persist while stdout is locked per-write

src/main.rs:

• added mod markdown, declaration

src/api/mod.rs: four changes

• replaced use tokio::io::{AsyncWriteExt, stdout} with use crate::markdown::MarkdownStream
• let ´mut stdout = stdout() replaced with let mut md = MarkdownStream::new() inside the loop so that parser state resets between tool-call rounds.
• replaced both stdout.write_all(...).await?; std.flush().await? sites with md.write_chink(...)?
• added md.finish()? call after the streaming loop (guarded by !json_mode)