Files view with editor and line comments in the right panel

[daniel-kurzynski]

Summary

Adds a Files view to the right panel and lets the user comment on lines of files and diffs, and on passages of chat messages. The comments go to the agent with the next message. The plan, including the decisions taken while building, is in docs/files-panel-plan.md.

Files view

• The session's project as a lazy tree. It loads folders when they are opened, hides gitignored entries unless switched on, and has a fuzzy name filter. The tree pane can be resized or hidden.
• The selected file opens in the code editor from gpui-component, with highlighting, line numbers and search. Edits save on their own about a second after typing stops, when another file is opened, and at once on Cmd/Ctrl-S. If the file changed on disk under unsaved edits, nothing is saved until the user picks Reload or Overwrite.
• A watcher keeps the tree and the open file current. The open file, expanded folders and the gitignore toggle are remembered per session.
• File access goes through new SessionService methods (list_project_dir, read_project_file, write_project_file, list_project_files), backed by fs_explorer::browse. Paths may not leave the project root, symlinks included.

Line comments

• Lines selected in the editor or in the Review diffs, and text selected in a chat message, can be commented. A small pill (copy, comment) appears at the end of the selection, and the comment card floats next to it. Cmd/Ctrl-Shift-M also works in the Files and Review views.
• A comment on a chat message carries the selected passage as a quote and reaches the model as <comment on="message"> with a <quote>. A message holding a pending comment's quote gets a border and a badge that reopens the comment.
• Commented lines are marked: tinted in the editor, where the tint follows edits, and with a bar in the Review diff. Diff rows now take presses through a hitbox, so the pill above them keeps its clicks.
• The composer shows all comments as one chip: a preview for one comment, a count for several. The chip lists the comments, jumps to them (in the panel or in the transcript) and removes them. Comments persist with the draft (DraftAttachment::LineComments).
• On send, the comments become one <line-comments> text block. Each comment carries its project-relative path, line range, the selected lines (long selections keep their ends) and the text. The transcript shows the block folded.
• After a load or save, a comment is found again by its excerpt, and its lines are updated when the code moved.

Other UI changes

• File paths in tool cards (file-edit card headers, read/search result headers) open the file in the Files view, at the shown line where there is one.
• The Review | Files | Browser switch moves into the title bar and replaces the panel toggle. A click opens the panel on that view or switches to it; clicking the shown view closes the panel.

Testing

• Unit tests: browsing, writing and watching in fs_explorer; the service methods; rendering, parsing and re-anchoring of line comments, including comments on messages; the fuzzy filter; line selection; mapping diff rows to line numbers; the comment chip label.
• Review view test: a selection shows the pill, and its comment button opens the card for the selected rows.
• Main screen tests: comments from the panel and from the transcript reach the draft, survive session switches and go with the next message; open-file requests show the file; the title bar switch opens, switches and closes the panel.
• cargo test, cargo clippy --all-targets --all-features -- -D warnings and cargo fmt --check pass.