go-live 9/9: write the v1 to v2 migration guide

[cliffhall]

Phase 9 of 9 in the v2 go-live runbook — see #1804 (§10). Pairs with #1803.

Write the v1 → v2 migration guide.

This is the highest-value doc shipped alongside the swap. CLI flags changed substantially, and --config semantics are genuinely different in v2 (read-only session file, with --catalog as the writable one). Without this, the deprecation notice sends people somewhere confusing.

Tasks

• CLI flag mapping, v1 → v2.
• --config vs --catalog semantics, with before/after examples.
• Node engine requirement change (>=22.7.5 → >=22.19.0).
• What no longer ships: the inspector-client / inspector-server / inspector-cli sub-packages (v2 is a single tarball).
• Link it from the deprecation notice, the 2.0.0 release notes, and the docs site (docs: new modelcontextprotocol.io Inspector documentation for v2 (legacy vs. modern era, more screenshots) #1803).