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/session/acp_session_tests/auth_error_no_retry_tests.rs

@@ -850,6 +850,7 @@ async fn set_session_model_invalidates_byok_memo_for_same_model_id() {
                 chat_compat: Default::default(),
                 auth_scheme: Default::default(),
                 anthropic_oauth: false,
+                github_copilot: false,
                 extra_headers: Default::default(),
                 context_window: 256_000,
                 force_http1: false,

crates/codegen/kigi-shell/src/session/acp_session_tests/cancel_running_task_tests.rs

@@ -47,6 +47,7 @@ async fn persist_ack_waits_for_disk_flush_before_success() {
                 chat_compat: Default::default(),
                 auth_scheme: Default::default(),
                 anthropic_oauth: false,
+                github_copilot: false,
                 extra_headers: Default::default(),
                 context_window: 100_000,
                 force_http1: false,
@@ -344,6 +345,7 @@ async fn first_turn_memory_injection_persists_to_chat_history() {
                 chat_compat: Default::default(),
                 auth_scheme: Default::default(),
                 anthropic_oauth: false,
+                github_copilot: false,
                 context_window: 100_000,
                 force_http1: false,
                 max_retries: None,
@@ -475,6 +477,7 @@ async fn first_turn_memory_injection_disabled_does_not_persist_to_chat_history()
                 chat_compat: Default::default(),
                 auth_scheme: Default::default(),
                 anthropic_oauth: false,
+                github_copilot: false,
                 context_window: 100_000,
                 force_http1: false,
                 max_retries: None,
@@ -1744,6 +1747,7 @@ async fn cancel_propagates_to_sampler_handle_so_no_further_emission() {
                 chat_compat: Default::default(),
                 auth_scheme: Default::default(),
                 anthropic_oauth: false,
+                github_copilot: false,
                 extra_headers: Default::default(),
                 context_window: 100_000,
                 force_http1: false,