TUI: make markdown links clickable via OSC 8 hyperlinks

[bug-ops]

Problem

Markdown links [text](url) in TUI chat messages are parsed by pulldown-cmark and styled (cyan + underline), but the URL is discarded after styling. The link text is visually distinct but not clickable in the terminal.

Bare URLs (e.g. https://github.com/...) are already clickable via regex detection + OSC 8 escape sequences in hyperlink.rs.

This means long URLs from LLM responses (PR links, documentation links, etc.) cannot be shortened to readable text like [PR #570](https://github.com/bug-ops/zeph/pull/570) — users either see the full URL or a styled but non-clickable label.

Current flow

pulldown-cmark parser
  → MdRenderer captures link_url in Event::Start(Tag::Link)
  → Text styled with theme.link (cyan + underline)
  → Event::End(TagEnd::Link) clears link_url
  → URL is discarded — never reaches hyperlink system

Proposed solution

Collect (rendered_text_range, url) pairs during markdown rendering and merge them into the hyperlink collection alongside regex-detected bare URLs.

Changes

1. widgets/chat.rs — MdRenderer

   • Add a field link_spans: Vec<(String, String)> to accumulate (display_text, url) pairs
   • On Event::End(TagEnd::Link), push the captured text and URL into link_spans
   • Expose link_spans via finish() return value (or a separate accessor)

2. widgets/chat.rs — render_md() / render_chat_message()

   • Propagate the collected link spans up to the caller
   • Store them alongside rendered lines so collect_from_buffer can use them

3. hyperlink.rs — collect_from_buffer()

   • Accept an additional parameter: pre-resolved markdown link mappings
   • For spans matching a known markdown link text at the expected position, use the stored URL instead of regex detection
   • Regex detection continues to handle bare URLs as before

4. No changes needed in write_osc8() — it already handles arbitrary HyperlinkSpan entries

Scope

• ~50-80 lines of changes across 2 files
• No new dependencies
• Backward compatible: bare URL detection unchanged

Acceptance criteria

• [text](url) in assistant messages renders as clickable OSC 8 hyperlink with text as display
• Bare URLs remain clickable as before
• Nested markdown formatting inside links preserved (e.g. [**bold link**](url))
• Existing hyperlink tests pass; new tests cover markdown link collection

Related files

• crates/zeph-tui/src/hyperlink.rs — URL detection, OSC 8 output
• crates/zeph-tui/src/widgets/chat.rs — MdRenderer, markdown rendering pipeline
• crates/zeph-tui/src/lib.rs — hyperlink emission in render loop