Make user turns visually distinct (heading + 👤 marker, opt-in color)

[huangruizhe]

Problem

In long transcripts it's hard to tell user turns from agent turns while scrolling — both render as the same italic-bold header. Yet user turns are the natural boundary between conversation rounds, so they're what you actually scan for.

Change

Render each user turn as a level-3 heading with a 👤 marker:

### 👤 User (2026-06-19 05:04:33Z)

• It pops visually and populates the document outline, so readers (VS Code, Obsidian, etc.) can jump between rounds.
• Agent turns stay quiet (italic-bold) with a 🤖 marker, so user turns remain the anchors:

  _**🤖 Agent (claude-... 2026-06-19 05:04:40Z)**_
  

• All plain Markdown — renders cleanly in every reader (GitHub, VS Code, Quick Look).

Opt-in color (off by default)

Markdown has no color of its own, and inline CSS is unreliable — VS Code's preview renders <span style="color:…">, but GitHub and macOS Quick Look strip/leak it. So color is opt-in via config, default off:

[markdown]
user_turn_color = "#2563eb"   # any CSS color; tints the user heading in readers that support inline HTML

When unset (default) the output is plain Markdown. When set, the user heading is wrapped in an inline-color <span>. This keeps the default portable while letting people who live in an HTML-capable reader opt into color.

Notes for downstream consumers

This changes the user/agent header format (_**User**_ → ### 👤 User, and _**Agent**_ → _**🤖 Agent**_). Anything that parses these headers to segment turns should match the new form (or both). Flagging explicitly since the .md is also consumed by tooling/indexers.

Tests

• New renderRoleHeader tests: user heading + 👤 marker, plain-by-default (no inline HTML), opt-in color span, agent stays quiet with 🤖.
• go build ./..., go test ./..., golangci-lint run ./... all clean.