feat: add opt-in compact folders to the file tree

[Jonathanm10]

Deep package hierarchies consume file-tree rows and push filenames far to the right. This adds compact_folders = true to join single-child directory chains into one row, with existing behavior preserved by default.

For a review containing app/src/main/kotlin/EditorScreen.kt:

Before                         With compact_folders = true
app/                           app/src/main/kotlin/
  src/                           EditorScreen.kt
    main/
      kotlin/
        EditorScreen.kt

Chains stop at directories containing direct review files or multiple child directories, using the active filtered file set. Each joined row expands or collapses as one unit. Full-path search, file identity, and diff navigation are preserved. Compact rows scroll by terminal columns, including wide and combining Unicode characters.

Addresses the compact-folder portion of #476. Flat mode, label ellipses, sidebar resizing, and runtime mode switching are outside this PR.

Validation

• cargo test: 1,618 passed, 4 ignored.
• cargo build, cargo check, and cargo fmt --check: passed.
• Dogfooded with cargo run -- -w against this diff. A live Kotlin fixture verified compact rendering, Enter collapse/reopen, and full-path search from a collapsed tree.
• Strict cargo clippy --all-targets -- -D warnings reports a pre-existing clippy::useless_format in src/forge/bitbucket/bkt.rs:1155, also present in base commit 58eb8e1. With that lint suppressed, all-target Clippy passes. That file is unchanged.

Related work

#370 proposes optional arrow navigation and scrolling changes; #556 changes file-tree scrolling and adds flat mode/resizing. Neither implements compact chains, but both touch related tree behavior and rendering.

[agavra]

Reviewed leniently for blockers. The branch is updated with current main, the README feature bullet was removed, and the merged tree passes formatting, Clippy, and the full test suite.