research: remove firecrawl_research_search_github from the search surface

Replaces the previous commit's approach. Rather than keeping the tool
registered with a deprecation notice, it is gone: agents fetch tools/list
when they start a session, so not listing it is the clearest signal there
is, and firecrawl_developer_search already covers the ground.

Removed with it: the GitHubItem type, the fmtGithub formatter and its one
constant, the allowlist entry, the instructions sentence, the README
bullet and the docs table row. The tool name moves to EXCLUDED_TOOLS in
the search profile test so it cannot leak back onto tools/list.

One consequence worth knowing: a client that already holds a session
keeps its cached tool list until it reconnects, so a call from such a
session fails with an unknown tool rather than migrating. New sessions,
which is most MCP traffic, never see it.

The agent-experience A/B harness still lists this name in exclude_tools.
Excluding a tool that is not registered is a no-op, so that contract
script keeps passing unchanged.

Removing a tool is a breaking change to the surface. The version bump is
left to the release, as usual for this repo.