feat(providers): add GitHub Copilot subscription OAuth (device flow + copilot-token re-mint)

28th platform `github-copilot` (uses_oauth, ChatCompletions wire). Two-stage auth:
RFC-8628 GitHub device flow (client Iv1.b507a08c87ecfe98, scope read:user, errors
in a 200 body) mints the DURABLE github token; a GET api.github.com/copilot_internal/
v2/token exchange re-mints the SHORT-LIVED copilot token. Persisted as key=copilot
token, refresh_token=github token, expires_at=copilot expiry; the "refresh" is a
copilot-token re-mint (not a refresh_token grant), dispatched via
OAuthTokenBody::GithubCopilotExchange in the generic refresher.

VS Code editor-identity headers on /models + /chat/completions, gated on
SamplerConfig.github_copilot / PlatformId::sends_copilot_editor_headers() so every
other ChatCompletions provider stays byte-identical. Live /models filtered
(parse_github_copilot_listing) to the openai-completions-served models: keep iff
model_picker_enabled && policy.state!="disabled" && tool_calls!=false AND not a
claude-4.x/5.x (messages) or gpt-5/oswe/mai- (responses-only) id — those need
per-model wire routing (documented debt), excluded rather than mis-routed.

Inherits the leak-safe pooled routing (scope oauth/github-copilot); its bearer/
refresh/api_key never touch the Kimi token (regression test added). Fail-fast on
an out-of-range copilot expires_at (would otherwise silently 401 mid-session).
Adversarial security review: GO, no CRITICAL/HIGH. Known limitation: Pi's
per-model policy-enablement POST is not ported (documented in AGENTS.md).

crates/codegen/kigi-shell/src/agent/subagent/mod.rs

@@ -888,6 +888,11 @@ async fn read_parent_sampling_config(
                     platform.oauth().is_some()
                         && platform.wire_api() == kigi_models::PlatformWireApi::Messages
                 });
+            // GitHub Copilot editor headers inherit from the parent model's
+            // platform (github-copilot → true); every other platform / BYOK →
+            // false, so the other ChatCompletions paths stay byte-identical.
+            let github_copilot = kigi_models::parse_managed_model_key(ctx.model_id.0.as_ref())
+                .is_some_and(|(platform, _)| platform.sends_copilot_editor_headers());
             let inherited = kigi_sampler::SamplerConfig {
                 api_key: creds.api_key,
                 base_url: cfg.base_url,
@@ -898,6 +903,7 @@ async fn read_parent_sampling_config(
                 api_backend: cfg.api_backend,
                 auth_scheme,
                 anthropic_oauth,
+                github_copilot,
                 chat_compat: cfg.chat_compat,
                 extra_headers,
                 context_window: cfg.context_window.get(),