fix(agent-gui): render pasted text mentions in conversation

[chovy-ai]

Summary

Long pasted text is stored as a canonical mention://pasted-text/... reference. The read-only rich-text conversation renderer did not recognize that built-in resource, so it fell back to ordinary text and exposed the complete Markdown URL in consuming applications such as TSH.

This change promotes pasted-text to a first-class Agent GUI mention, preserves its href/path through parsing and attribute round-trips, and renders it through the existing file MentionPill presentation. It also adds parser and read-only conversation regression coverage to ensure the canonical URL is no longer shown.

Verification

• pnpm --filter @tutti-os/agent-gui exec vitest run --environment jsdom --maxWorkers=3 shared/AgentRichTextReadonly.spec.tsx agent-gui/agentGuiNode/agentRichText/agentFileMentionExtension.spec.ts — 2 files and 56 tests passed, including the pasted-text conversation regression.
• pnpm --filter @tutti-os/agent-gui test — 246 files and 2092 tests passed.
• pnpm --filter @tutti-os/agent-gui typecheck — passed.
• pnpm check:changed --tail-lines 100 — 11 lanes passed; the pre-push push-ready check passed 12 lanes.

Checklist

• This PR does not change agent lifecycle semantics.
• I kept the change focused on one concern.
• I checked documentation impact; the existing large-paste and read-only-rendering architecture documentation already describes the intended behavior.
• No README or CONTRIBUTING language variants are changed.
• I ran the lowest meaningful local checks for the changed surface.
• My commit is signed off with DCO.