refactor(cli): use errore for typed error handling in run command

Replace untyped catch blocks and scattered process.exit() with
errore tagged errors and error-as-values pattern. Compile-time
safety for error handling paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

PLAN.md

@@ -0,0 +1,90 @@
+# Proactive Error Handling with errore in `run.ts`
+
+Ref: https://errore.org/
+
+## Goal
+
+Replace scattered `process.exit()`, untyped `catch (e: any)`, and inline error handling in `run.ts` with errore's error-as-values pattern. Compile-time exhaustive error handling via `matchError` in a single top-level `action()`.
+
+## Constraints
+
+- `makeAppFromEnv` and `makeConfigApp` are exported — **do not change their return types**
+- `platform.ts` (`startServer`, `serveStatic`, `getConfigPath`) — **unchanged this PR**
+- Keep it simple: errors defined in `run.ts`, not a separate file
+
+## Phase 1: Add errore dependency
+
+- `app/package.json`: add `"errore"` to dependencies
+- Run install
+
+## Phase 2: Define tagged errors in `run.ts`
+
+```ts
+import errore from "errore"
+
+class ConfigLoadError extends errore.createTaggedError({
+  name: "ConfigLoadError",
+  message: "Failed to load config: $reason",
+}) {}
+
+class NeedsBunError extends errore.createTaggedError({
+  name: "NeedsBunError",
+  message: "Config requires Bun runtime",
+}) {}
+
+class NeedsTypeStrippingError extends errore.createTaggedError({
+  name: "NeedsTypeStrippingError",
+  message: "Node $version needs --experimental-strip-types for .ts config",
+}) {}
+
+class ReexecFailedError extends errore.createTaggedError({
+  name: "ReexecFailedError",
+  message: "Re-exec failed: $reason",
+}) {}
+```
+
+## Phase 3: Refactor internal functions to return errors
+
+### `loadConfigFile` → `ConfigLoadError | NeedsBunError | NeedsTypeStrippingError | CliBkndConfig`
+- Use `errore.tryAsync` to wrap `import()`
+- On catch: detect Bun requirement → `NeedsBunError`
+- On catch: other failure → `ConfigLoadError`
+- Before import: check `needsTypeStripping()` → `NeedsTypeStrippingError`
+
+### `reexecWithTypeStripping` / `reexecUnderBun` → `ReexecFailedError | never`
+- Success: `process.exit(0)` (unavoidable — must stop parent after child completes)
+- Failure: return `ReexecFailedError` instead of `process.exit(1)`
+- Caller decides how to handle failure
+
+### `makeAppFromEnv` internal flow (return type unchanged)
+- Internally use error-as-values from `loadConfigFile`
+- On `NeedsBunError` / `NeedsTypeStrippingError`: call re-exec, handle `ReexecFailedError`
+- On `ConfigLoadError`: `console.error` + `process.exit(1)`
+- Keep exported signature: `Promise<App>` (exits on unrecoverable errors)
+
+### `action()` — cleaner top-level
+- `makeAppFromEnv` handles its own error cases internally (since we can't change its return type)
+- `action()` stays thin: call `makeAppFromEnv` → `startServer`
+
+## Phase 4: Verify
+
+- No behavior change for end users
+- All `catch (e: any)` replaced with typed errore patterns
+- `process.exit(1)` consolidated to fewer, explicit locations
+- Re-exec failure paths return typed errors before exiting
+
+## Files Changed
+
+| File | Change |
+|---|---|
+| `app/package.json` | add `errore` dep |
+| `app/src/cli/commands/run/run.ts` | tagged errors, refactored error flow |
+
+## Commit Message
+```
+refactor(cli): use errore for typed error handling in run command
+
+Replace untyped catch blocks and scattered process.exit() with
+errore tagged errors and error-as-values pattern. Compile-time
+safety for error handling paths.
+```