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-shell/src/agent/models_fetch.rs

@@ -176,8 +176,9 @@ fn fetch_platform_models_blocking(
     let enabled = enabled_platforms(auth.is_some(), platform_keys);
     if enabled.is_empty() {
         return Err(BackendError::Auth(
-            "No platform credentials: log in with `kigi login` or configure a moonshot API key \
-             (KIGI_MOONSHOT_API_KEY or [platforms.*] in ~/.kigi/config.toml)."
+            "No platform credentials: log in with `kigi login`, paste a platform API key in \
+             the login screen (stored in ~/.kigi/auth.json), or set a platform env var such \
+             as KIGI_MOONSHOT_API_KEY."
                 .into(),
         ));
     }