feat(tui): render structured mentions inline

[kitlangton]

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.