feat(provider-adapter): implement multi-provider abstraction layer (Phase 1)

Add provider types, registry, capabilities, and message adapter registry.
Rename sdk-messages→claude-messages, agent-runner→claude-runner,
ClaudeSession→AgentSession. Update Rust AgentQueryOptions with provider
and provider_config fields. Capability-driven AgentPane rendering.

v2/src/lib/providers/claude.ts

@@ -0,0 +1,20 @@
+// Claude Provider — metadata and capabilities for Claude Code
+
+import type { ProviderMeta } from './types';
+
+export const CLAUDE_PROVIDER: ProviderMeta = {
+  id: 'claude',
+  name: 'Claude Code',
+  description: 'Anthropic Claude Code agent via SDK',
+  capabilities: {
+    hasProfiles: true,
+    hasSkills: true,
+    hasModelSelection: true,
+    hasSandbox: false,
+    supportsSubagents: true,
+    supportsCost: true,
+    supportsResume: true,
+  },
+  sidecarRunner: 'claude-runner.mjs',
+  defaultModel: 'claude-sonnet-4-20250514',
+};

v2/src/lib/providers/registry.svelte.ts

@@ -0,0 +1,26 @@
+// Provider Registry — singleton registry of available providers (Svelte 5 runes)
+
+import type { ProviderId, ProviderMeta } from './types';
+
+const providers = $state(new Map<ProviderId, ProviderMeta>());
+
+export function registerProvider(meta: ProviderMeta): void {
+  providers.set(meta.id, meta);
+}
+
+export function getProvider(id: ProviderId): ProviderMeta | undefined {
+  return providers.get(id);
+}
+
+export function getProviders(): ProviderMeta[] {
+  return Array.from(providers.values());
+}
+
+export function getDefaultProviderId(): ProviderId {
+  return 'claude';
+}
+
+/** Check if a specific provider is registered */
+export function hasProvider(id: ProviderId): boolean {
+  return providers.has(id);
+}

v2/src/lib/providers/types.ts

@@ -0,0 +1,34 @@
+// Provider abstraction types — defines the interface for multi-provider agent support
+
+export type ProviderId = 'claude' | 'codex' | 'ollama';
+
+/** What a provider can do — UI gates features on these flags */
+export interface ProviderCapabilities {
+  hasProfiles: boolean;
+  hasSkills: boolean;
+  hasModelSelection: boolean;
+  hasSandbox: boolean;
+  supportsSubagents: boolean;
+  supportsCost: boolean;
+  supportsResume: boolean;
+}
+
+/** Static metadata about a provider */
+export interface ProviderMeta {
+  id: ProviderId;
+  name: string;
+  description: string;
+  capabilities: ProviderCapabilities;
+  /** Name of the sidecar runner file (e.g. 'claude-runner.mjs') */
+  sidecarRunner: string;
+  /** Default model identifier, if applicable */
+  defaultModel?: string;
+}
+
+/** Per-provider configuration (stored in settings) */
+export interface ProviderSettings {
+  enabled: boolean;
+  defaultModel?: string;
+  /** Provider-specific config blob */
+  config: Record<string, unknown>;
+}