TUI: support `@file` mentions in the input bar to attach files/dirs to the next message

[Hmbown]

Motivation

Recent session pain: user knew exactly which file they wanted in context (`docs/DeepSeek_V4.pdf`) but had to describe it and let the agent search. Standard practice in Claude Code, Cursor, Codex CLI, opencode is `@` autocomplete in the input — type `@`, get a fuzzy file picker, hit enter, the file's path (and optionally contents) are attached to the next user message.

Behavior

• Typing `@` in the input bar pops a fuzzy file picker rooted at the workspace.
• Selection inserts the relative path as e.g. `@docs/DeepSeek_V4.pdf` into the input text (visible to the user).
• On submit, the harness expands each mention into either:
  • Path-only mode (default): appends a "# Attached files" block listing the absolute paths so the model can choose to read them.
  • Inline mode (when small): auto-includes the file contents up to a per-file budget (~10K tokens). PDFs use `pdftotext` if available, else fall back to path-only with a note.
• `@` followed by a directory inserts the dir; on expand we attach a one-level tree listing.
• Multiple `@` mentions in one message all expand.

Implementation pointers

• Input bar lives in `crates/tui/src/tui/app.rs` / the input handling code there.
• Expansion happens in the message-submit path — find where `Op::SendUserMessage` (or equivalent) is built before sending to the engine.
• Fuzzy matcher: there's already `utils::project_tree` (used in `prompts.rs:54`); pair with `fuzzy-matcher` crate for picker.
• For PDF: shell out to `pdftotext` if on PATH; otherwise attach "<binary file: $PATH>".

Out of scope

• Auto-attaching by detecting bare paths in plain text. Only `@`-prefixed mentions get expanded.
• Image attachment / VLM (separate feature; can build on top later).

Acceptance criteria

• Typing `@` opens picker; arrow keys + enter select; esc cancels.
• Picker is fast in repos with >10k files (debounced search, capped result list).
• Submitted message visibly contains `@` so the user can see what was attached.
• Tab-completion works on partial paths.

Target

v0.5.1 (basic path-only) — inline expansion can slip to v0.5.2.

[Hmbown]

Shipped in v0.5.1 via commit d79178a: typing @<partial> and pressing Tab now resolves the mention against the workspace via ignore::WalkBuilder, picks unique matches, extends to longest common prefix on multiple, and surfaces ambiguous candidates in the status line. The mention-expansion path that ships file contents to the model is unchanged — this is purely a discovery aid for typing the path.

The @-popup picker and inline-contents auto-expansion stay queued for v0.5.2 — opening as a separate issue if you want a tracking item for those. Closing this one as the v0.5.1 acceptance (path-only basics) is met.