feat(tui): render structured mentions inline

Problem

Structured references are styled while composing a prompt, but projected user messages render the original text as plain text and show attachments separately below it.

For example, a selected skill currently becomes:

Compare the workspace provider options. /api-design

[ skill | API design ]

The same underlying inconsistency exists for file and agent mentions: composer extmarks communicate structure inline, but timeline rendering loses that styling and relies on separate badges or previews.

Proposed direction

Introduce one shared user-message mention renderer that segments message text using the structured attachment mention ranges and styles each recognized reference inline:

• @file references
• @agent references
• /skill references

Render a separate badge or preview only when the attachment has no valid inline mention or needs richer presentation, such as an image preview.

renderMentions(message.text, {
  files: message.files,
  agents: message.agents,
  skills: message.skills,
})

The renderer should validate that each stored range still matches its mention text and fall back safely when ranges are stale or overlap.

Scope

This is a follow-up to #41188. That PR keeps the existing timeline convention and adds structured skill attachments; it should not special-case skill rendering before files and agents share the same treatment.

Acceptance criteria

• Structured file, agent, and skill mentions retain recognizable inline styling after submission.
• Inline mentions and separate badges are not duplicated unnecessarily.
• Attachments without valid mention ranges still render through the existing badge/preview UI.
• Multiple mentions, Unicode text, stale ranges, and overlapping ranges have focused tests.

Start by locating the projected user-message timeline renderer and the composer’s structured attachment mention handling, then trace how files, agents, and skills are represented by their stored ranges. Use the acceptance criteria to guide focused tests for multiple mentions, Unicode text, stale ranges, and overlaps. Done means valid mentions are styled inline without unnecessary duplicate badges, while invalid or richer attachments retain the existing badge or preview UI.