feat(tui): add /context inventory and /drop to remove attached files

[Sayt-0]

Closes #3435

Summary

Adds an Aider-style context inventory to the TUI: a /context dialog listing every file currently entering the session context (attachments and prompt files) with per-item token estimates, plus the ability to drop attachments, interactively or via /drop <path>.

Issue expectations mapping

Issue expectation	Implemented
List attached files and prompt files with a per-item token estimate	/context dialog with "Attached files" and "Prompt files (agent config)" sections, ~N tokens per file (size/4, the same rule of thumb as the session truncation budget), plus a totals line
Allow removing an attachment from the session	d/x/delete/backspace on the selected entry, or /drop <path> directly; /drop without argument opens the dialog
Removal API on Session.AttachedFiles	Session.RemoveAttachedFile(absPath) bool, mutex-guarded, next to AddAttachedFile
Keep @ref completion consistent (editor.go)	No editor change needed: @ completion lists workspace files (session-independent); re-mentioning a dropped file re-attaches it through the existing send path

Flow

/context ──► ShowContextDialogMsg ──► Session.AttachedFilesSnapshot()
                                      + App.CurrentAgentPromptFiles()   (local runtime only)
                                  ──► NewContextDialog(BuildContextFiles(...))

d (dialog) ─┐
/drop path ─┴► DropAttachedFileMsg ──► expand ~/$VAR + filepath.Abs
                                   ──► Session.RemoveAttachedFile()
                                   ──► SessionStore.UpdateSession()
                                   ──► notification toast

Notes

• Prompt files are listed but not droppable: they are re-resolved from the agent config at every turn (add_prompt_files hook), so dropping one is refused with an info toast.
• Prompt-file paths are resolved with the same lookup rules as the turn-start hook (promptfiles.PathsFromEnv), exposed through an optional runtime capability; remote runtimes simply omit the section.
• Files deleted since attachment render as missing (no token estimate).
• Dropped entries are persisted via SessionStore.UpdateSession so resumed sessions do not resurrect them.
• Long lists use a sliding window that keeps the selection visible.

Testing

• Unit tests: Session.RemoveAttachedFile (3 cases) and the context dialog (8 cases: entry building, empty state, sections/token rendering, navigation bounds, drop semantics, prompt-file refusal, selection clamping, close).
• go build ./... and golangci-lint run: clean.
• Full go test ./...: green except TestURLSource_Read_RejectsLocalAddresses (pkg/config), which fails identically on main in this environment (network-dependent, unrelated to this change).