feat(config)!: one ordering for configuration sources, and a bootstrap deny rule

$DSH_HOME/.env had just become an ordinary environment layer, which left the
harness resolving user-facing values from a flattened process.env that could
no longer say where a value came from. A key stored through the web page
stayed shadowed by an older key in the user's own .env. An endpoint could be
redirected by the project: the invoking directory's .env is materialized like
every other layer, and a base URL decides where a resolved API key is sent, so
a DEEPSEEK_BASE_URL written into a model-editable workspace would send the
user's credential — and the prompts carrying their code — to whatever host
that file named.

Give every user-facing value one ordering, with four kinds of source:

  explicit for this run     per-operation override, CLI argument
  > authored by deployment  --config / --config-replace
  > this launch's shell     inherited process environment
  > product-managed store   settings.yaml, .credentials.yaml
  > discovered file         $DSH_HOME/.env
  > defaults                schema default, shipped base, public default

The domains differ only in which tiers exist. The earlier split — credentials
ranking the environment over the managed file while settings ranked over the
environment — was inconsistent: the distinguishing fact is who authored the
source, not the domain.

packages/util/environment owns an immutable snapshot with per-layer
provenance. getFrom(name, sources) searches only the layers a caller names,
and omitting one is a refusal rather than a demotion: the adapters ask for
['process', 'user-env'], so no reordering can let a project file back into a
decision it was excluded from.

isBootstrapOnly rejects, before anything is materialized, any .env setting a
variable that governs how a process launches (PATH, SHELL, NODE_OPTIONS,
LD_PRELOAD), where code or model-visible instructions load from (the whole
DSH_* namespace, HOME, XDG_*), or how the network is reached (proxy and CA
variables). The namespace is denied wholesale so a switch added later cannot
become settable by being forgotten, and there is no opt-out.

verify-config-source-ownership keeps both rules: no unregistered process.env
read under packages/*/*/src (26 allowlisted with reasons), and no apiKey,
baseURL, or headers inlined from the environment in shipped Cordis config —
removing those inlines is what makes the deployment tier meaningful.

packages/credentials/credentials-local/tests/local.spec.ts

@@ -4,6 +4,7 @@ import { mkdir, mkdtemp, readFile, rm, stat, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join, resolve } from 'node:path'
 import { credentialRef } from '@deepseek-ai/dsh-credentials'
+import { createEnvironmentSnapshot, DSH_ENVIRONMENT_KEY } from '@deepseek-ai/dsh-environment'
 import type { CredentialRef } from '@deepseek-ai/dsh-credentials'
 import { CredentialsLocal, resolveSpec } from '../src/index.ts'
 
@@ -100,6 +101,74 @@ describe('layering and reads', () => {
   })
 })
 
+describe('layer ladder', () => {
+  // inherited process env > .credentials.yaml > $DSH_HOME/.env, and the
+  // invoking directory's .env supplies no credential at all.
+  async function bootLayered(
+    path: string,
+    layers: Parameters<typeof createEnvironmentSnapshot>[0],
+  ): Promise<Context> {
+    const ctx = new Context()
+    ctx.provide(DSH_ENVIRONMENT_KEY, createEnvironmentSnapshot(layers))
+    const fiber = ctx.plugin(CredentialsLocal, { path, watch: false })
+    cleanups.push(async () => { await fiber.dispose() })
+    await fiber
+    return ctx
+  }
+
+  it('lets the stored value beat the user .env, so a UI write takes effect immediately', async () => {
+    const dir = await tempDir()
+    const path = join(dir, '.credentials.yaml')
+    await writeFile(path, 'DSH_CRED_TEST: stored\n')
+    const ctx = await bootLayered(path, [
+      { source: 'process', values: {} },
+      { source: 'user-env', path: '/home/.dsh/.env', values: { DSH_CRED_TEST: 'older-user-env' } },
+    ])
+    expect(await ctx.credentials.resolve(KEY)).toEqual({ value: 'stored', source: 'file' })
+    // The old dead end is gone: a key sitting in the user's .env no longer
+    // makes the stored one unwritable.
+    expect(await ctx.credentials.describe(KEY)).toEqual({ configured: true, source: 'file', writable: true })
+    await expect(ctx.credentials.set(KEY, 'rotated')).resolves.toBeUndefined()
+    expect(await ctx.credentials.resolve(KEY)).toEqual({ value: 'rotated', source: 'file' })
+  })
+
+  it('serves the user .env only when nothing is stored', async () => {
+    const dir = await tempDir()
+    const ctx = await bootLayered(join(dir, '.credentials.yaml'), [
+      { source: 'process', values: {} },
+      { source: 'user-env', path: '/home/.dsh/.env', values: { DSH_CRED_TEST: 'from-user-env' } },
+    ])
+    expect(await ctx.credentials.resolve(KEY)).toEqual({ value: 'from-user-env', source: 'user-env' })
+    // Writable: storing a key replaces it as the effective one.
+    expect(await ctx.credentials.describe(KEY)).toEqual({ configured: true, source: 'user-env', writable: true })
+  })
+
+  it('ignores the invoking directory .env entirely', async () => {
+    const dir = await tempDir()
+    const ctx = await bootLayered(join(dir, '.credentials.yaml'), [
+      { source: 'process', values: {} },
+      { source: 'project-env', path: '/work/.env', values: { DSH_CRED_TEST: 'from-project' } },
+    ])
+    // A project directory can be written by the model, and a substituted key
+    // would route every request through an account someone else reads.
+    expect(await ctx.credentials.resolve(KEY)).toBeUndefined()
+    expect(await ctx.credentials.describe(KEY)).toEqual({ configured: false, writable: true })
+  })
+
+  it('lets only the inherited environment shadow the store, read-only', async () => {
+    const dir = await tempDir()
+    const path = join(dir, '.credentials.yaml')
+    await writeFile(path, 'DSH_CRED_TEST: stored\n')
+    const ctx = await bootLayered(path, [
+      { source: 'process', values: { DSH_CRED_TEST: 'from-shell' } },
+      { source: 'user-env', path: '/home/.dsh/.env', values: { DSH_CRED_TEST: 'from-user-env' } },
+    ])
+    expect(await ctx.credentials.resolve(KEY)).toEqual({ value: 'from-shell', source: 'env' })
+    expect(await ctx.credentials.describe(KEY)).toEqual({ configured: true, source: 'env', writable: false })
+    await expect(ctx.credentials.set(KEY, 'next')).rejects.toThrow(/launching environment/)
+  })
+})
+
 describe('document validation', () => {
   // Every rejection below is a boot failure rather than a skipped entry: this
   // document holds nothing but credentials, so an ignored key would read as