feat(files): drag-and-drop rename / move in the Files tree

[danielss-dev]

Summary

• Repo::move_path (crates/strand-core/src/rename.rs): rename / move a working-tree entry. Tracked sources (any index entry at or under the path) shell out to git mv so the index entry moves with the file — staged content preserved, directory moves and case-only renames handled natively; untracked sources are a plain fs rename. Refuses to overwrite, creates missing destination parent dirs, and guards both ends (safe_workdir_path for the source, an ancestor-walking variant for the not-yet-existing destination).
• repo_move_path IPC (quick sync write) + useRepo.moveEntries: sequential moves with a single snapshot refresh, per-entry failure strings so one collision does not hide the moves that succeeded; an open file view follows its file to the new path.
• PierreTree drag-to-move (opt-in onMove): pointer-based — the rows live in Pierre's shadow root where nothing can be marked draggable, but mouse events compose across the boundary. All imperative refs + direct DOM so the 60Hz mousemove never re-renders the tree. 5px threshold, cursor-chasing ghost chip naming entry + target (dashed when invalid), --bg-sel wash on the hovered folder row, file rows target their containing folder, bare tree space the repo root, Escape cancels, a multi-selected file drags the whole selection.
• RenameFileDialog behind the Files-tree context menu ("Rename / move…") for keyboard parity; filename segment preselected.
• Docs: ROADMAP 1.0 items closed (drag-and-drop renames + the 100k-commit perf pass line, which the 0.5 perf close already satisfied), TASKS entries added.

Test plan

• cargo test -p strand-core — 96 pass (+5 new rename tests: tracked/untracked/directory moves, overwrite + missing + no-op guards, traversal/absolute rejection)
• cargo clippy --workspace --all-targets — no new warnings (4 pre-existing in commit.rs/history.rs test code from a newer clippy)
• tsc --noEmit, vitest run (198 pass), vite build
• End-to-end over WebView2 CDP against the running app: dragged a tracked root file into src/ (git status R rootfile.txt -> src/rootfile.txt), an untracked file into docs/ (stays untracked, index untouched), and dialog-renamed a modified file (RM with the worktree edit preserved); asserted the ghost chip text mid-drag and its removal after drop.

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