The native new-tab list rendered one generic line ("opens in the DSH
sidebar") under every plugin entry, so six plugin rows read identically.
`TabDescriptor.description` now carries the real purpose (string or
locale-following thunk; absent falls back to the generic line), and the
six visible builtins declare theirs across all 20 dictionaries.
- service.ts: `TabDescriptor.description` + docs (guide §4.1, §0 row).
- native/: the guide entry's description follows the descriptor; the
`files` takeover reuses the editor's line.
- builtins: files / changes / tasks / side chat / terminal / browser.
- tests: builtins assert every visible type declares a distinct line;
the lifecycle test asserts descriptor lines (and the takeover reuse).