feat(credentials): move the store to .credentials.yaml and layer $DSH_HOME/.env

$DSH_HOME/.env carried two incompatible jobs. As credentials-local's writable
secret store it could not be hoisted into process.env — hoisting makes every
stored key read as a read-only launch override and blocks rotation from the
TUI and the web page. But its name and dotenv format promise an environment
file, so a DEEPSEEK_BASE_URL sitting beside a working DEEPSEEK_API_KEY in the
same file was silently ignored: only the credential provider read the
document, and it addresses credential references alone.

Split the two jobs into two files.

.credentials.yaml is the provider-managed store: a strict YAML mapping of
CredentialRef to non-empty string, no version field, no wrapper level. Because
it holds credentials and nothing else, a non-mapping root, a non-identifier
key, a non-string value, an empty string, a duplicate key, and malformed YAML
are all rejections rather than skipped entries — loud at boot and at a write,
warn-and-keep-last-good on a live reload. The dotenv physical-line editor
gives way to a patch of the parsed document, so comments and untouched entries
keep their formatting and any string value round-trips, multi-line included.
Writer lock, read-modify-write, atomic 0600 write under a 0700 directory,
watcher, self-write suppression, and quiescent disposal are unchanged.

$DSH_HOME/.env becomes the user's ordinary environment layer. app-boot's new
loadLayeredEnv loads the invoking directory's .env then the Harness home's,
giving user < project < inherited; the home resolves from the inherited
environment first, so a project .env cannot redirect it.

Credential precedence is unchanged: the live environment still wins read-only
over the file, and shadowed writes still reject. Whether a provider-managed
store should instead win over the environment is a separate decision.

No migration: a key already in $DSH_HOME/.env keeps resolving through the new
environment layer, as a read-only env source that shadows the stored one.

apps/web/tests/models-settings.e2e.ts

@@ -81,7 +81,7 @@ describe('web e2e: Models settings page configures a dormant provider', () => {
     await dialog.getByLabel('API 密钥').fill('sk-e2e-minimax')
     await dialog.getByRole('button', { name: '保存', exact: true }).click()
     // The profile lands in settings.yaml with only the derived reference, the
-    // key value lands in the harness home's .env, the dormant route
+    // key value lands in the harness home's .credentials.yaml, the dormant route
     // registers, and the topology frame invalidates the page into the row.
     const row = dialog.getByText('minimax-cn', { exact: true }).first()
     await row.waitFor({ timeout: 10_000 })
@@ -89,8 +89,8 @@ describe('web e2e: Models settings page configures a dormant provider', () => {
     expect(document).toContain('minimax-cn:')
     expect(document).toContain('apiKeyEnv: MINIMAX_CN_API_KEY')
     expect(document).not.toContain('sk-e2e-minimax')
-    const stored = await readFile(join(scaffold.harnessHome, '.env'), 'utf8')
-    expect(stored).toContain('MINIMAX_CN_API_KEY=sk-e2e-minimax')
+    const stored = await readFile(join(scaffold.harnessHome, '.credentials.yaml'), 'utf8')
+    expect(stored).toContain('MINIMAX_CN_API_KEY: sk-e2e-minimax')
     expect(await page.content()).not.toContain('sk-e2e-minimax')
   }, 60_000)
 
@@ -136,8 +136,8 @@ describe('web e2e: Models settings page configures a dormant provider', () => {
       async () => readFile(join(scaffold.harnessHome, 'settings.yaml'), 'utf8'),
       { timeout: 10_000 },
     ).not.toContain('minimax-cn:')
-    expect(await readFile(join(scaffold.harnessHome, '.env'), 'utf8'))
-      .toContain('MINIMAX_CN_API_KEY=sk-e2e-minimax')
+    expect(await readFile(join(scaffold.harnessHome, '.credentials.yaml'), 'utf8'))
+      .toContain('MINIMAX_CN_API_KEY: sk-e2e-minimax')
     await expect.poll(
       async () => page.getByRole('dialog', { name: '删除模型提供方？' }).count(),
       { timeout: 10_000 },