move the feature catalogue out of docs, and replace the queue with a plan

Two structural changes, both about what an agent may pull into context.

exploration.md catalogues engine features nobody has asked for. That is storage,
not working material, so it moves to ideas/ where nothing sweeps it and it is
opened only when named — the same rule the other parked material already
follows. Six references repointed; the ideas gates then demanded an index line
and a status, and both were supplied rather than exempted.

build-queue.md was 516 lines, nearly all of it entries 0-23 finished months of
work ago, with the plan buried at the top. It becomes .scratch/continue.md at
49: where the code is, what is planned, and the findings worth carrying that no
doc owns — chiefly that silent damage to prose is this engine's recurring
failure mode, and that three defects this arc were invisible to curl.

Docs and HARNESS point at the new names. No rule, gate or threshold changed.

docs/extensions.md

@@ -86,7 +86,7 @@ tested with an untrusted-input case.
    there is real logic.
 5. Failure degrades: a broken extension logs and is skipped, never takes a request down.
 6. Needing a permanent external service or a primitive change makes it a **trunk** — see
-   `exploration.md`.
+   `ideas/exploration.md`.
 7. One directory, no sibling imports, and a `doc.go` in this shape (ADR-0027):
 
 ```go