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.

packages/llm/llm-pi-ai/tests/dynamic-config.spec.ts

@@ -44,7 +44,7 @@ async function boot(dir: string, config: LlmPiAi.Config): Promise<Context> {
   })
   await ctx.plugin(LlmService)
   await ctx.plugin(SettingsLocal, { path: join(dir, 'settings.yaml'), watch: false })
-  await ctx.plugin(CredentialsLocal, { path: join(dir, '.env'), watch: false })
+  await ctx.plugin(CredentialsLocal, { path: join(dir, '.credentials.yaml'), watch: false })
   await ctx.plugin(LlmPiAi, config)
   return ctx
 }
@@ -53,7 +53,7 @@ describe('request-level dynamic profiles', () => {
   it('mounts bare and dormant, then registers routes the moment settings supply providers', async () => {
     vi.stubEnv('PI_DYNAMIC_KEY', '')
     const dir = await home()
-    await writeFile(join(dir, '.env'), 'PI_DYNAMIC_KEY=pk-from-settings\n')
+    await writeFile(join(dir, '.credentials.yaml'), 'PI_DYNAMIC_KEY: pk-from-settings\n')
     const server = await mockServer([{ events: textEvents }])
     // The exact product posture: `- id: llm-pi-ai` with no config at all.
     const ctx = await boot(dir, {})
@@ -112,7 +112,7 @@ describe('request-level dynamic profiles', () => {
   it('rotates the per-request credential referenced by apiKeyEnv', async () => {
     vi.stubEnv('PI_DYNAMIC_KEY', '')
     const dir = await home()
-    await writeFile(join(dir, '.env'), 'PI_DYNAMIC_KEY=pk-one\n')
+    await writeFile(join(dir, '.credentials.yaml'), 'PI_DYNAMIC_KEY: pk-one\n')
     const server = await mockServer([{ events: textEvents }, { events: textEvents }])
     const ctx = await boot(dir, {
       providers: { deepseek: { apiKeyEnv: 'PI_DYNAMIC_KEY', baseURL: server.url } },

packages/llm/llm-pi-ai/tests/loader-composition.spec.ts

@@ -3,7 +3,7 @@
  * settings-local, credentials-local, and a bare `llm-pi-ai` row boot from a
  * test-only cordis.yml through the actual Loader + Include path, an external
  * edit of settings.yaml registers the route live, and the next request
- * carries the credential the .env supplies. A hand-mounted `ctx.plugin` cannot
+ * carries the credential the credentials document supplies. A hand-mounted `ctx.plugin` cannot
  * catch Loader export-shape failures, which is why the twin adapter has the
  * same guard.
  */
@@ -40,7 +40,7 @@ async function loadComposition(): Promise<{ ctx: Context; settingsPath: string }
   root = await mkdtemp(join(tmpdir(), 'dsh-pi-composition-'))
   const settingsPath = join(root, 'settings.yaml')
   await writeFile(settingsPath, '# personal settings\n')
-  await writeFile(join(root, '.env'), 'PI_COMPOSITION_KEY=key-from-store\n')
+  await writeFile(join(root, '.credentials.yaml'), 'PI_COMPOSITION_KEY: key-from-store\n')
 
   const configPath = join(root, 'cordis.yml')
   await writeFile(configPath, [
@@ -54,7 +54,7 @@ async function loadComposition(): Promise<{ ctx: Context; settingsPath: string }
     '- id: credentials',
     "  name: '@deepseek-ai/dsh-credentials-local'",
     '  config:',
-    `    path: ${JSON.stringify(join(root, '.env'))}`,
+    `    path: ${JSON.stringify(join(root, '.credentials.yaml'))}`,
     '    debounceMs: 10',
     '- id: llm-pi-ai',
     "  name: '@deepseek-ai/dsh-llm-pi-ai'",