Add per-provider auth.json keys; make auth methods registry-generic (P0b)

Platform API keys now live in auth.json under the platform-id scope (the
per-provider auth.json key contract), resolved env > auth.json > legacy
[platforms.*] config.toml (read-only fallback). The TUI login picker,
paste box, auth-method advertising, and authenticate handler are all
registry-generic: a new PlatformSpec row appears in the login UI and
authenticates with zero UI changes. Spec rows gained vendor/console_host/
login_label display fields (moonshot strings byte-identical, pinned by
tests).

Adversarial review caught that auth.json keys were validated at login but
never stamped onto catalog entries (completions would 401; restart lost
eager auth). Fixed red-green: resolve_model_list/resolve_model_catalog now
take a resolved PlatformApiKeys snapshot consumed by the credential-
stamping layer (auth.json beats stale config.toml, matching the login
validator), with production callers resolving fresh per catalog build.
Also from review: the new auth.json writer takes the manager's cross-
process flock (bounded retry — an unlocked RMW racing a token refresh
could revert a rotated refresh token); the oauth-401 wiremock test is
hermetic (KIGI_SHARE_DIR tempdir; it could read a dev's real auth.json
and hit live moonshot); cli_models resolves real keys; auth.json is read
once per registry sweep; caller-less lock_config_writes deleted; catalog
resolvers tightened to pub(crate); stale config.toml doc comments and the
no-credentials error copy updated.

crates/codegen/kigi-tui/src/app/dispatch/auth.rs

@@ -290,7 +290,7 @@ pub(super) fn dispatch_cancel_platform_key_entry(app: &mut AppView) -> Vec<Effec
 }
 
 /// Enter with a non-empty key in the API-key paste box: persist the key to
-/// `[platforms.<id>]` in config.toml, then authenticate with the platform's
+/// auth.json (platform-id scope), then authenticate with the platform's
 /// method id (one sequential background task — see the effect handler).
 /// The screen shows the connecting state while the key is validated; a
 /// failure lands back on the picker with the error line (`AuthFailed`).