Add scan roots and an interactive directory picker to the TUI

[UnbreakableMJ]

Rebased onto main after #3 merged. Base is now main; the diff is exactly this round's changes.

Summary

The TUI could only ever scan $HOME — vacuum-tui called safety::default_roots() directly and vacuum_tui::run() took no arguments, so a target outside $HOME (e.g. /spacecraft-software) was reachable only via a HOME=… hack (which also misdirects the XDG trash). This makes the TUI take roots properly and adds a runtime directory picker.

• CLI roots. vacuum tui [PATHS]... [--root PATH]... now accepts roots (cli::TuiArgs) and resolves them through the same machinery as scan/list/clean: app::run_tui reuses resolve_roots + config::fallback_roots, so precedence is CLI --root/paths > VACUUM_ROOTS > config roots > $HOME. Bare vacuum/vacuum tui still defaults to $HOME but now also honors VACUUM_ROOTS/config. run(roots) threads the roots into both the scan and the Deleter.
• Interactive picker. Press r to open a Clear-based centered overlay that browses the filesystem (std::fs only — no new deps) and adds/removes scan roots at runtime, re-scanning on close. Keys: j/k move · enter/l descend · h/Backspace up · space add/remove (✓ marks active roots) · d remove · esc apply. Pure helpers (read_subdirs, toggle_root) carry the logic; symlinks are not followed and unreadable dirs are skipped.
• Safety unchanged. The picked roots become the deletion allowlist via safety::check_deletable exactly as for the CLI — the picker cannot widen the boundary, and the system-path denylist still applies.

Surface & docs

• schema/describe advertise tui (with paths + root); stale "Defaults to $HOME" descriptions corrected to the real precedence.
• Texinfo manual: new Interactive TUI chapter (keys + picker), builds zero-warning.
• AGENTS.md and SKILL.md updated.

Testing

• cargo fmt --check · cargo clippy --workspace --all-targets -- -D warnings · cargo test --workspace (44 pass — 2 new CLI parse tests, 3 new TUI unit tests) · reuse lint compliant · makeinfo zero-warning.
• Reinstalled to ~/.cargo/bin/vacuum; verified tui --help/schema/describe from outside the repo.
• Caveat: the interactive event loop needs a TTY and is not unit-tested — coverage lives in the pure helpers and arg parsing. The picker UI warrants a manual terminal smoke-test.

Follow-ups (out of scope)

• Persisting picker-added roots back to the config file (one-shot per session for now).
• Per-root scan progress for very large trees.

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