[b-re-w]

A Colab account declared before 3.0 with tailcat_binary = "tailcat-disabled-for-this-account" was refused whole by 3.0.2, with a message telling a Colab user to set up a tunnel agent. Both are wrong: that field carries no connection intent, and the agent migration applies to the tunnel kind only.

Spec change first, in docs/SPEC.md Connection strategies and Agent and pairing item 14.

Behaviour after this change, in Provider.init:

• tailcat_binary, any kind: ignored, one stderr line per process per alias saying it is obsolete since 3.0 and can be deleted from the config file. Not a fallback, because the field selects nothing in 3.0.
• tailcat and tailcat_port, tunnel account: refused with the agent migration text (letify agent serve, then letify login tunnel ALIAS --connect letify-agent2:TOKEN).
• tailcat and tailcat_port, any other kind: refused with 'obsolete since 3.0, delete it from ALIAS in the config file', no agent text.

No other removed field exists in the code: a grep for auto_install and QUIC account options found none.

Red then green: tests/test_tailcat_removed.py had 17 failures before the code change (per-kind messages, one notice per alias for all five kinds) and 30 passes after. Full suite: 2286 passed, 7 skipped. ruff check letify tests is clean. Guides and READMEs in English and Korean updated.