Closes #3435. Builds on the /context dialog from #3432: an Aider-style inventory to see which files are in context and remove them.

Changes

Attached files
▶ main.go      2.1K    2%  /proj/main.go
  readme.md    300    <1%  /proj/docs/readme.md
  deleted.txt  -       -   /tmp/deleted.txt (missing)

Prompt files
  AGENTS.md    600    <1%  /proj/AGENTS.md

Behavior notes

• Per-file estimates mirror the send pipeline: inlined text uses the calibrated heuristic, supported binaries use the flat attachment charge, oversized or unsupported files show -. They detail content already counted in Messages, so the estimated total is unchanged.
• Missing files stay listed and droppable, so dangling references can be cleaned up.
• Dropping stops propagation to sub-agents and skill prompts; content already inlined in past messages stays until compaction (a note in the dialog says so). Re-attaching via @ or /attach works as before.
• Prompt files are config-driven and re-injected every turn, so they are listed read-only.

Testing

• Unit tests: session removal, breakdown inventory (text, binary, missing, oversized), dialog rendering, navigation and drop, /drop parsing, App.DropAttachedFile resolution.
• go build ./..., full test suite, golangci-lint run and go run ./lint . are clean.