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/ui/app-boot/README.md

@@ -8,6 +8,7 @@ Shared boot glue for the app bins ([`dsh`](../../../apps/cli/README.md), [`dsh-c
 |---|---|
 | `resolveConfigPath(path, snapshotMode, cwd?)` | Absolute config path; `snapshotMode === 'replay'` swaps a `cordis.yml`/`.yaml` basename for its sibling `cordis.snapshot.yml` |
 | `loadEnv(binName, dir?, warn?)` | Load the gitignored `.env` (Node `process.loadEnvFile`); absent file is fine, an unloadable one warns a single labelled line (default: stderr) |
+| `loadLayeredEnv(binName, cwd?, warn?)` | The `dsh` product CLI's user environment: `loadEnv` over the invoking directory, then over the Harness home, giving `user < project < inherited`. The home is resolved from the inherited environment first, so a project `.env` cannot redirect it |
 | `installFailLoud(binName, proc?, release?)` | Turn an unhandled boot or later Loader rejection into one labelled stderr line + `exit(1)`; the optional `release` teardown is awaited between the two (bounded by `FAIL_LOUD_RELEASE_TIMEOUT_MS`) so a terminal-owning surface restores the terminal before exit; returns the uninstaller (for tests) |
 | `FAIL_LOUD_RELEASE_TIMEOUT_MS` | How long `installFailLoud` waits for its `release` hook; a wedged disposer delays the fatal exit, never cancels it |
 | `assertEntriesLoaded(ctx, binName)` | Throw when a settled tree holds an enabled entry with no fiber, reporting every unresolved plugin name as a Cordis startup failure |
@@ -33,7 +34,7 @@ This package carries no loader hooks and no dev-mode surface. The [`dsh` app](..
 
 A developer's machine-local preferences live outside every repository in the Harness home (default `~/.dsh`, overridable via `$DSH_HOME`; the single root [`resolveDshHome`](../../util/paths/README.md) resolves), consumed by the `dsh` CLI's TUI, Web, and headless surfaces ([`apps/cli`](../../../apps/cli/README.md)); the demo bins boot their committed trees verbatim. Two optional files:
 
-- **`.env`** — the credential store of [`dsh-credentials-local`](../../credentials/credentials-local/README.md), read by that provider alone. No surface hoists it into `process.env`: doing so would make every stored key look like a read-only launch override on the next run, blocking rotation from the TUI and the web page. The environment layers are the ambient one and the invoking directory's `.env` (loaded by the bin; `process.loadEnvFile` never overrides), and a composition without the credential provider keeps resolving keys from those alone.
+- **`.env`** — the user's ordinary environment layer, loaded by the `dsh` bin through `loadLayeredEnv` beneath the invoking directory's `.env` and the inherited environment. It is plain environment with plain environment reach, not a secret boundary: what the Harness owns and isolates lives in `.credentials.yaml`, which no surface hoists. A key placed in this file therefore still resolves — as a read-only `env` layer that shadows the stored one and blocks rotation from the TUI and the web page.
 - **`config.yaml`** — loader overlay patches applied over the shipped default config, with the same semantics as the shipped surface overlays: an id-targeted patch replaces the named entry's whole `config` (restate unchanged fields), `insert` adds entries, and `!!js` expressions interpolate at mount. A patch naming an entry id absent from the booted tree is a silent no-op. An empty or comments-only file throws (it parses to nothing, not to a list); disable the overlay with `[]` or by deleting the file.
 
 The TUI and Web keep `config.yaml` live through `watchPersonalPatches`; one-shot headless runs read only the startup value. The watcher targets the exact personal path even when the file or immediate parent does not exist, serializes bursts, and recomposes the personal patches inside the caller's layer order (surface overlay below, app-generated patches above). A rejected read, parse, or Loader candidate leaves the last good tree running and the HMR service broadcasts `hmr/config-update-failed(filename, Error)` after logging it; observer failures are contained. Disposing the context closes the watcher and drains an active refresh.
@@ -52,5 +53,5 @@ No direct invalidation from `boot()`; a consumer that calls `addHarnessSourceSec
 
 - **Bare package specifiers depend on Loader internals** — production bins need Loader's optional native helper; an in-process caller without it must use resolvable relative/file specifiers or provide its own module-resolution hook.
 - **Snapshot replay swapping is basename-specific** — only a config ending in `cordis.yml` or `cordis.yaml` maps to the sibling `cordis.snapshot.yml`; custom config names require caller-managed selection.
-- **Environment loading is cwd-scoped and optional** — the helper loads one `.env` file and warns on failure; it does not search parents, merge profiles, or validate required variables.
+- **Environment loading is directory-scoped and optional** — each layer is one named directory's `.env`, and a failure warns; neither helper searches parents or validates required variables. `loadLayeredEnv` fixes its two layers at the invoking directory and the Harness home, so a caller wanting different layers composes `loadEnv` itself.
 - **Personal config is patch-shaped** — an id-targeted patch replaces the entry's whole `config` rather than deep-merging, so a personal override restates the base fields it keeps.