docs(maestro): add the command bar contract

Specifies the ctrl-k command bar for both clients: opening and the
leader escape hatch for a literal Ctrl-K, sigils, empty-query recency,
the key model including the session action page, and touch entry.

The match-and-rank algorithm is pinned exactly — normalization, word
boundaries, the per-character score, embedding choice, tie-break — and
closed with twelve fixtures the TypeScript (#146) and Rust (#147)
matchers are both tested against, so their orders cannot drift.

CLAUDE.md

@@ -87,6 +87,11 @@ or is upgraded under them — the build stamp on `/api/info`, the status
 indicator's states, the blocking overlay, grace and poll timings. Read it
 before changing reconnect behaviour on either client.
 
+`docs/COMMAND-BAR.md` is the contract for the `ctrl-k` command bar in both
+clients — how it opens, the sigils, the key model, the match-and-rank algorithm
+the two matchers must agree on, and the fixture table they are tested against.
+Read it before changing either bar or either matcher.
+
 `nix run .#dev` (or `just dev`) brings up the whole local stack under
 process-compose: a debug daemon on 127.0.0.1:7071, then Vite on :5173 once the
 daemon answers `/api/info`. The SPA proxies `/api/*` back to the daemon, so only