feat(workspace): support Markdown and text attachments

[tlysanhuo]

Closes #170

Summary

• accept .md, .markdown, and .txt files from the composer picker, paste, and drag-and-drop paths
• infer Markdown/text MIME types from file extensions and render them as file attachment cards
• decode text/* data URLs into message text so every model can consume the contents without native file support

Verification

• bun test test/session/message-v2.test.ts test/session/prompt-attachments.test.ts (25 pass)
• bun typecheck (7 workspace packages pass)
• playwright test e2e/prompt-attachments.spec.ts --project=chromium (1 pass)
• prettier --check on changed files

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