feat(tui): render headless session conversation history

[edwin-zvs]

Problem

Headless sessions (zarvis, claude, codex, antigravity — any harness run with --mode headless) emit their conversation as structured Message/Reasoning/ToolUse/ToolResult events with no PTY. The TUI renders the selected session via ItemHistory.replay, which only ingested PTY bytes and tool blocks — so a headless session showed its tool blocks (and (no PTY history yet …)) but never the assistant prose. The webui already renders the full transcript in chat mode; the TUI didn't.

Approach (2): a first-class message item

• Item::Message { kind, text, break_before } added to ItemHistory (MessageKind = User / Assistant / Reasoning).
• feed_message coalesces the adapter's per-delta Message/Reasoning stream into contiguous runs. It pushes one item per delta (append-only) so replay stays incremental — mutating one growing item would force a full vt100 reparse per delta (the old "typing lag" failure mode). Only the first item of a run sets break_before.
• synth_message renders role-styled bytes (assistant plain, user gray ❯, reasoning dim), CRLF-normalized, through the existing replay_full vt100 interleaving path. The fast/shadow-scrollback paths now defer to replay_full whenever any non-PTY item exists.

Harness-agnostic gating

Wiring is gated on is_headless(session) (mode == "headless"), which mirrors the webui's isHeadlessSession exactly — so it covers headless zarvis/claude/codex/antigravity uniformly (shell is never headless). Applied in both:

• the live event handler, and
• transcript replay — apply_transcript_to_local_state gains an is_headless arg, threaded through SessionHydrationRequest (derived from the session summary's mode).

PTY-backed (interactive) sessions keep their prose in the PTY stream and are left untouched — no double-rendering.

Tests

• feed_message_coalesces_runs_and_marks_breaks — run coalescing + break_before flags + empty-delta ignore.
• headless_message_history_renders_prose_and_tool_blocks — assistant prose interleaved with tool blocks renders (no PTY).
• reasoning_and_user_messages_render — user ❯ + reasoning + assistant.
• transcript_replay_renders_messages_only_when_headless — replay folds Message/Reasoning into history when headless, and ignores them when PTY-backed.
• Full agentd-cli suite (169) green; --locked workspace build clean.

Notes / styling choices (flag if you'd prefer otherwise)

• Assistant = plain prose, user = gray ❯ prompt, reasoning = dim. A single \r\n separates message runs / tool blocks (mirrors the tool-block layout).
• Whether a headless session emits user-prompt Message events depends on the adapter; this renders whatever roles arrive.

🤖 Generated with Claude Code