offweb: gemini, gopher and ipfs fetch for agents

The two tools serve completely different purposes: `about` returns operator/identity/limits metadata, while `fetch` retrieves a URL over gemini/gopher/ipfs/ipns. There is no plausible scenario where an agent would confuse one for the other.

`fetch` is a bare verb and `about` is a noun/preposition-style endpoint, so the two names do not follow a shared verb_noun convention. Both are individually conventional and readable, but the set is mixed rather than consistent.

With only two tools, the surface is borderline thin, but the server's scope is genuinely narrow: one fetch operation plus an identity/limits endpoint. It earns its minimalism, though it sits at the low end of the appropriate range.

The core domain—fetching text from non-HTTP networks and returning a verifiable receipt—is fully covered, and `about` documents limits and the public key. Minor gaps exist, such as no batch/multi-URL fetch and no built-in receipt verification tool, but these are workable externally.