feat(credentials): abstract credential seam (ctx.credentials)

References-not-values doctrine: settings carry env-shaped CredentialRefs,
providers own storage. Per-operation resolve, UI-safe describe, fail-loud
set/unset under read-only shadowing, credentials/updated commit event with
a live-service invariant.

packages/credentials/credentials/tests/invariant.spec.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import InvariantService from '@deepseek-ai/dsh-invariants'
+import { credentialRef } from '../src/index.ts'
+import * as CredentialsInvariant from '../src/invariant.ts'
+import { MemoryCredentials } from './memory.ts'
+
+const REF = credentialRef('DEEPSEEK_API_KEY')
+
+describe('credentials invariant companion', () => {
+  it('accepts a committed change emitted by a live service', async () => {
+    const ctx = new Context()
+    await ctx.plugin(InvariantService)
+    await ctx.plugin(CredentialsInvariant)
+    await ctx.plugin(MemoryCredentials)
+
+    await expect(ctx.credentials.set(REF, 'sk-live')).resolves.toBeUndefined()
+  })
+
+  it('fails an update event emitted without a live service', async () => {
+    const ctx = new Context()
+    await ctx.plugin(InvariantService)
+    await ctx.plugin(CredentialsInvariant)
+
+    expect(() => { ctx.emit('credentials/updated', REF) }).toThrow(/invariant violated by "@deepseek-ai\/dsh-credentials"/)
+  })
+
+  it('reserves the package name against duplicate registration', async () => {
+    const ctx = new Context()
+    await ctx.plugin(InvariantService)
+    await ctx.plugin(CredentialsInvariant)
+
+    expect(() => {
+      ctx.invariants.register('@deepseek-ai/dsh-credentials', () => {})
+    }).toThrow(/already registered/)
+  })
+})