fix(messaging): pass through PDF attachments

[huntharo]

Summary

• Pass PDF attachments from messaging platforms through as bounded file inputs instead of extracting text from them.
• Preserve Codex compatibility by writing file inputs to a local temp path and sending Codex a file reference/mention rather than an unsupported raw file input variant.
• Render file parts in desktop transcripts and carry PDF/file metadata through the agent-core message builder.

Replaces closed PR #386.

Verification

• pnpm test apps/desktop/src/main/tests/codex-client.test.ts apps/desktop/src/main/tests/messaging-attachment-processor.test.ts apps/desktop/src/main/tests/messaging-controller.test.ts
• pnpm --filter @pwragent/desktop typecheck
• pnpm lint:boundaries

Notes

• The original PR fix(messaging): pass through PDF attachments #386 was closed unmerged and GitHub refused to reopen it, so this recreates the PR from the same branch.