Remove EQL v2 installation and Proxy lifecycle tooling from stash and migrate

[tobyhede]

Part of #778 and the EQL v2 removal.

Remove the remaining EQL v2 installation and rollout leaf from packages/cli and packages/migrate.

Acceptance criteria:

• Delete vendored EQL v2 SQL, v2 install routing, Proxy configuration lifecycle commands, cutover implementation, and obsolete flags including eql-version and latest.
• Keep eql upgrade on its v3-only default path.
• Retain legacy v2 status/read diagnostics, manifest fallbacks needed to display existing deployments, and the rewrite token needed for old migration history.
• Reject removed v2 CLI options clearly and point dump-recovery users to the upstream EQL 2.3.1 SQL release.
• Update affected skills, command manifest coverage, tests, and changesets in the same change.

This is the CLI/migrate removal track; it must not remove v2 ciphertext read compatibility.