fix(app): harden Halos workflow polish

[fschrhunt]

What changed

• Adds persistent local tombstones for deleted Halos sessions so a deleted session stays hidden after app restart even if stale session list data comes back.
• Broadens composer drag/drop and paste intake so file URLs, file-like text, images, PDFs, HTML, RTF, and other supported media attach through the composer path instead of becoming raw text.
• Improves plugin/tool lifecycle copy for future plugin surfaces and keeps plugin panels ending before final assistant text.
• Refines closed-sidebar title animation and workbench resizing/divider behavior.

Verification

• swift test
• ./script/build_and_run.sh --verify

Risk

Attachment handling now accepts a wider set of paste/drop types. Manual checks should include dragging files onto the chat body, dragging onto the composer field, and pasting copied screenshots/media.