fix(mentions): preserve icons in readonly conversations

[jomeswang]

Summary

• hydrate readonly rich-text mentions through the same trigger provider resolveMention contract used by the composer
• render shared MentionPill tokens in readonly content with workspace app, issue, session, and file semantics
• replace failed custom icon images with semantic fallback glyphs in UI System and AgentGUI surfaces
• keep icon presentation out of serialized mention Markdown

Why

Workspace apps already resolve icon presentation while composing a mention, but app-owned conversation history only retained durable mention identity. The generic readonly renderer did not invoke the provider resolver, so sent messages lost their app icons. Separately, a non-empty but broken icon URL suppressed fallback glyphs in multiple shared surfaces.

Verification

• pnpm check:changed
• pnpm check:ui-boundaries
• UI System tests and typecheck
• UI rich-text: 94 tests and typecheck
• AgentGUI: 192 files, 1793 tests and typecheck
• repository check:full preflight passed; its parallel Go lane hit unrelated timeout and goal-state flakes, and every reported failing Go test passed immediately when rerun individually

Release

Includes a changeset for agent-gui, ui-rich-text, and ui-system.