feat(identity): add handle/DID resolution service

Provides isHandle/isDid validation re-exports from @atproto/syntax and
a resolveHandleOrDid function that uses the public Bluesky API to
resolve handles to DIDs and vice versa. Foundation for profile CRUD.

src/services/identity.ts

···
+import { isValidHandle, isValidDid } from '@atproto/syntax';
+import { Agent } from '@atproto/api';
+
+export const isHandle = isValidHandle;
+export const isDid = isValidDid;
+
+export async function resolveHandleOrDid(
+  handleOrDid: string,
+): Promise<{ did: string; handle: string }> {
+  const agent = new Agent('https://public.api.bsky.app');
+
+  if (isDid(handleOrDid)) {
+    const profile = await agent.getProfile({ actor: handleOrDid });
+    return { did: profile.data.did, handle: profile.data.handle };
+  }
+
+  if (isHandle(handleOrDid)) {
+    const resolved = await agent.resolveHandle({ handle: handleOrDid });
+    return { did: resolved.data.did, handle: handleOrDid };
+  }
+
+  throw new Error(`Invalid handle or DID: ${handleOrDid}`);
+}

tests/services/identity.test.ts

···
+import { describe, it, expect } from 'vitest';
+import { isHandle, isDid } from '../../src/services/identity.js';
+
+describe('Identity utilities', () => {
+  it('identifies handles', () => {
+    expect(isHandle('alice.bsky.social')).toBe(true);
+    expect(isHandle('did:plc:abc123')).toBe(false);
+  });
+
+  it('identifies DIDs', () => {
+    expect(isDid('did:plc:abc123')).toBe(true);
+    expect(isDid('alice.bsky.social')).toBe(false);
+  });
+});