Drop the Welcome page's key tip rather than deriving it

As a derived keys row it gave Welcome a row with a description, and
harness/tests/help.spec.js holds Welcome to prose; CI failed on it at
544744c. Help mode's status hint already derives next-page.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/test-evidence/protocol.md

@@ -1041,7 +1041,13 @@ command name. At HEAD the test failed naming all four.
 
 The fix is structural rather than a second scanner. The three prefix rows are
 ordinary `keys(..)` rows now — `keys` also asks `Keymap::prefixed_keys_running`
-for two-key sequences — and the tip is a `keys` row in `ModeKey::Help`.
+for two-key sequences. The tip is **gone** rather than derived: made a `keys` row
+in `ModeKey::Help`, it gave the Welcome page its one row with a description,
+and `harness/tests/help.spec.js` ("? opens help, and its rows carry the keys and
+the words") failed in CI on `welcome.every(row => row.description === null)` —
+Welcome is prose by contract. The Help mode's own status hint already derives
+`next-page` from the keymap, so the literal was a second table (improvise-rr8n's
+rule: keep the one derived from the thing it describes).
 `literal()` and `dim()` are gone; the only way left to build a
 `HelpLine::Literal` is the `ex!` macro, which writes the leading colon itself, so
 what remains hand-written is ex-command text — checked against the parser by