Add per-message timestamps to markdown and HTML output

[madorian]

Summary

The JSONL source data already contains ISO 8601 timestamps on every message entry (entry.get("timestamp")), and extract_conversation() correctly preserves them in the conversation dicts. However, save_as_markdown() and save_as_html() only use the first message's timestamp for the file-level header metadata -- individual message headings have no temporal information.

This PR appends the formatted timestamp to each message heading:

• Markdown: ## 👤 User [2026-05-18 21:00:05]
• HTML: inline <span> after the role label, styled in subdued gray

When a message has no timestamp field, the heading renders exactly as before (no brackets), maintaining backward compatibility.

Motivation

Per-message timestamps enable correlating conversation events with external systems (git commits, file modifications, CI logs) without parsing raw JSONL. This is especially valuable for forensic analysis across concurrent sessions.

Changes

• Added _format_timestamp() static method for consistent ISO-to-display formatting
• Updated save_as_markdown() to include [timestamp] on each role heading
• Updated save_as_html() to include timestamp span on each role heading

Example output

## 👤 User [2026-05-18 21:00:00]

Hello

---

## 🤖 Claude [2026-05-18 21:00:05]

Hi there

---

## 👤 User

Message without timestamp (renders cleanly)

[ArchiLantern]

Great PR! This per-message timestamp is very helpful.

One suggestion to make it even better: add timezone conversion for users outside UTC+0.
For example, a --timezone Asia/Shanghai (or --offset +8) option that displays timestamps in local time.
The JSONL stores UTC, but many users (like me) want to see Beijing time directly in the exported markdown/HTML.

Would you be open to adding that, or should I file a separate issue for it?