catalogue: add io.pilot.plainweb v1.0.0

[Alexgodoroja]

Adds io.pilot.plainweb v1.0.0 to the prod app-store catalogue.

Plainweb is an HTTP adapter (no CLI binary) that retrieves any web page as clean Markdown — no HTML, no JS — via a single plainweb.fetch(url) call.

What's included

• catalogue/catalogue.json — new io.pilot.plainweb entry (4-platform bundles, R2 prod URLs + sha256, metadata pointer, publisher pin)
• catalogue/catalogue.json.sig — re-signed with the release key
• catalogue/apps/io.pilot.plainweb/metadata.json — catalogue-v2 metadata (long description)

Bundles are hosted on R2 prod (pilot-artifacts-prod) for all 4 platforms (darwin/linux × arm64/amd64), each verified HTTP 200.

Verified

• Built from the merged app-template submission with the stable publisher key; PIN ed25519:9oZGhTSuJJ5xaePW89I9QSOnyp8p83igvGj0jEUuLoE=
• appstore install against the hosted bundle → sha256 OK
• plainweb.fetch {"url":"https://example.com"} via the installed app → returned {content_type:"text/markdown; charset=utf-8", content:"…"}
• Catalogue signature verifies and plainweb lists under the release pin