refactor(coding-agent): replace AgentSessionRuntimeHost with closure-based AgentSessionRuntime

- Replace AgentSessionRuntimeHost and bootstrap abstractions with AgentSessionRuntime
- Runtime creation is now closure-based via CreateAgentSessionRuntimeFactory
- Factory closes over process-global fixed inputs, recreates cwd-bound services per effective cwd
- Session config (model, thinking, tools, scoped models) re-resolved per target cwd
- CLI resource paths resolved once at startup as absolute paths
- Swap lifecycle: teardown old, create next, apply next (hard fail on creation error)
- Unified diagnostics model (info/warning/error) for args, services, session resolution, resources
- No logging or process exits inside creation/parsing logic
- Removed session_directory support
- Removed session_switch and session_fork extension events (use session_start with reason)
- Moved package/config CLI to package-manager-cli.ts
- Fixed theme init for --resume session picker
- Fixed flaky reftable footer test (content-based polling)
- Fixed silent drop of unknown single-dash CLI flags
- Added error diagnostics for missing explicit CLI resource paths
- Updated SDK docs, examples, plans, exports, tests, changelog

fixes #2753

packages/coding-agent/src/core/extensions/index.ts

@@ -116,9 +116,6 @@ export type {
 	SessionBeforeTreeEvent,
 	SessionBeforeTreeResult,
 	SessionCompactEvent,
-	SessionDirectoryEvent,
-	SessionDirectoryHandler,
-	SessionDirectoryResult,
 	SessionEvent,
 	SessionShutdownEvent,
 	// Events - Session

packages/coding-agent/src/core/extensions/types.ts

@@ -441,12 +441,6 @@ export interface ResourcesDiscoverResult {
 // Session Events
 // ============================================================================
 
-/** Fired before session manager creation to allow custom session directory resolution */
-export interface SessionDirectoryEvent {
-	type: "session_directory";
-	cwd: string;
-}
-
 /** Fired when a session is started, loaded, or reloaded */
 export interface SessionStartEvent {
 	type: "session_start";
@@ -522,7 +516,6 @@ export interface SessionTreeEvent {
 }
 
 export type SessionEvent =
-	| SessionDirectoryEvent
 	| SessionStartEvent
 	| SessionBeforeSwitchEvent
 	| SessionBeforeForkEvent
@@ -921,16 +914,6 @@ export interface BeforeAgentStartEventResult {
 	systemPrompt?: string;
 }
 
-export interface SessionDirectoryResult {
-	/** Custom session directory path. If multiple extensions return this, the last one wins. */
-	sessionDir?: string;
-}
-
-/** Special startup-only handler. Unlike other events, this receives no ExtensionContext. */
-export type SessionDirectoryHandler = (
-	event: SessionDirectoryEvent,
-) => Promise<SessionDirectoryResult | undefined> | SessionDirectoryResult | undefined;
-
 export interface SessionBeforeSwitchResult {
 	cancel?: boolean;
 }
@@ -1006,7 +989,6 @@ export interface ExtensionAPI {
 	// =========================================================================
 
 	on(event: "resources_discover", handler: ExtensionHandler<ResourcesDiscoverEvent, ResourcesDiscoverResult>): void;
-	on(event: "session_directory", handler: SessionDirectoryHandler): void;
 	on(event: "session_start", handler: ExtensionHandler<SessionStartEvent>): void;
 	on(
 		event: "session_before_switch",