Add Disk Usage plugin

[charpeni]

What

A new Disk Usage plugin: see what's taking up disk space on the bb server host, ncdu-style. Scan a directory, get its immediate children sorted by recursive size, and drill down until you find the culprit.

• Disk Usage panel — scans a directory (default: the server home directory) and lists children largest-first with share bars, breadcrumbs, click-to-drill-down, and a free-form path input. While a scan runs, a live progress card streams entries visited, bytes so far, and the directory currently being walked (over bb.realtime).
• Per-path cache — the last result per path is kept in memory (LRU-capped), so drilling back up or reopening the panel is instant; the header shows the snapshot's age and Rescan forces a fresh walk. Concurrent requests for the same path join a single walk.
• bb disk-usage CLI — the same scan for agents and scripts: bb disk-usage [path] [--top N] [--json] [--refresh].

How sizes are measured

• Allocated disk blocks (blocks × 512), not apparent size, so sparse files report what they actually occupy.
• Symlinks are never followed; hardlinked files are counted once per scan.
• Unreadable entries are skipped and counted, never fatal.
• Directories are walked concurrently (bounded semaphore), and a scan visits at most 500,000 entries — past that it stops descending and flags the result as truncated, with the budget spread across the tree instead of exhausted depth-first.

Testing

• 7 unit tests via createFakePluginHost covering recursive sizes, cache/refresh semantics, concurrent-scan dedupe, realtime progress signals, and CLI output (human, cached, JSON, and error paths).
• pnpm run validate, typecheck, lint, fmt:check, and bb plugin build all pass.
• Exercised live: installed as a path plugin, verified the panel (cached instant load, live progress card, drill-down) and the CLI against a real ~500k-entry home directory.

Not yet listed in marketplace.json — that needs a disk-usage/v0.1.0 release tag, which can follow once this merges.