feat(cli): /files shows files referenced in the session

[emal-avala]

Summary

Replaces the previous stub /files (which just prompted the agent to `ls` the cwd) with a real command that surfaces every file the session has touched — @-mentioned paths, FileRead, FileWrite, FileEdit, MultiEdit, NotebookRead, NotebookEdit.

> /files
Files referenced this session (3):

  src/main.rs          [@,read  ] ×3
  src/lib.rs           [edit    ] ×1
  README.md            [@       ] ×1

Each entry shows which access modes were used (@ = @-mention, read = FileRead, write = FileWrite, edit = FileEdit/MultiEdit/NotebookEdit) and the total reference count.

Why

Mid-session, the user often can't remember which files the agent has looked at. Scrolling back through the transcript is slow and noisy. /files gives a dense summary in one place — useful for context hygiene, cost audits, and writing PR descriptions.

Implementation

Pure-read over engine.state().messages:

• User messages → regex-lite extraction of @path mentions (same rule as the REPL's @-expansion: @ at start or after whitespace, path contains / or .)
• Assistant messages → scan ContentBlock::ToolUse for file-accessing tools (FileRead, FileWrite, FileEdit, MultiEdit, NotebookRead, NotebookEdit) with path / file_path / notebook_path input fields

Dedup by path; sources are set-merged so the same file with multiple access modes shows all of them. Sorted alphabetically.

New public API:

pub enum FileSource { Mention, Read, Write, Edit }
pub fn collect_session_files(&[Message]) -> Vec<(String, Vec<FileSource>, usize)>

Test plan

• cargo fmt --all — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test -p agent-code --bin agent -- extract_at_mentions extract_tool_file collect_session_files — 6/6 pass
  • mention extraction: simple, email-rejection, path-shape-requirement
  • tool-input extraction: known tools mapped correctly, unknown tools skipped
  • end-to-end aggregation: mixed user+assistant transcript, set-merged sources

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