[Feat] Cross-task mention and artifact reference via @ picker

[samzong]

Summary

Replace the single-purpose FilePicker with a tab-based MentionPicker that supports two mention types: Tasks (injects full conversation context excluding tool calls) and Files (workspace artifacts from the file manager). Type @ in the chat input to open the picker, then use Tab to switch sections, arrow keys to navigate, and Enter to select.

Type of change

• [Feat] new feature
• [UI] UI or UX change

Why is this needed?

Users need to reference other task conversations and workspace files as input context when composing messages. Previously, the @ picker only supported files from manually-added context folders. This replaces it with a unified mention system that can reference tasks (pulling their full conversation history) and artifacts from the file management system.

What changed?

• New MentionPicker.tsx: Tab-based picker component (Tasks | Files) with keyboard navigation hints, artifact loading from fileStore, and extensible TABS array for future mention types
• Replaced FilePicker.tsx: Deleted — all functionality migrated to MentionPicker
• ChatInput.tsx rework: New selectedTasks/selectedArtifacts state; unified commitMention() handler; handleSend loads full conversation history for mentioned tasks (user+assistant messages, no tool calls) with 500KB size guard; artifact content read via readArtifactFile; default tab based on artifact availability

Architecture impact

• Owning layer: renderer
• Cross-layer impact: none (uses existing IPC: loadMessages, readArtifactFile, listArtifacts)
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: no changes to message persistence writers

Validation

• pnpm typecheck
• pnpm test
• pnpm build
• Manual smoke test
• Not run

pnpm typecheck → ok (no errors)
pnpm test → 19 files, 82 tests passed

Release note

• User-facing change. Release note is included below.

You can now type @ in the chat input to mention other tasks or workspace files. Task mentions inject the full conversation context; file mentions embed artifact content. Use Tab to switch between Tasks and Files, arrow keys to navigate, Enter to select.

Checklist

• The PR title uses at least one approved prefix: [Feat]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

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