bug(mecatui): assistant-response Markdown links are not terminal-clickable

Summary

Markdown links in mecatui assistant responses are not actionable in terminals that normally support terminal links, including iTerm2 and Ghostty. For example, a rendered [PR #812](https://github.com/stacklok/mecatl/pull/812) cannot be opened with Cmd-click, right-click, or equivalent terminal link actions.

Expected behavior

Assistant-response Markdown links should be terminal-clickable, preferably through validated OSC 8 hyperlinks where the terminal supports them. Plain URLs should also remain detectable by terminals where applicable.

Scope and safety constraint

This should not weaken the terminal-control sanitization used for untrusted plain UI content (tool output, permission prompts, errors, etc.). That boundary is intentional to prevent terminal-escape/UI-spoofing injection.

Relevant code

• Assistant Markdown rendering: cmd/mecatui/ui/render.go (markdownWidth, Glamour renderer)
• Markdown link styling: cmd/mecatui/theme/glamour.go
• Plain-content terminal safety boundary: cmd/mecatui/ui/sanitize.go

The status-line protocol intentionally does not emit OSC 8 yet; this issue concerns assistant response rendering rather than StatusML links.

Start in cmd/mecatui/ui/render.go at the markdownWidth and Glamour rendering path, then read cmd/mecatui/theme/glamour.go for link styling. Compare the assistant-response path with the safety boundary in cmd/mecatui/ui/sanitize.go; done means Markdown links are actionable in supported terminals while untrusted plain UI content remains sanitized.