fix: validate sandbox plugin exports and fix plugin packaging (#363)

* fix(webhook-notifier): add build step and export built files

The webhook-notifier plugin exported raw TypeScript source from its
package.json exports (./sandbox pointed to src/sandbox-entry.ts).
When the Vite plugin resolved this at site build time, it embedded
unbuilt TypeScript into the sandbox module, causing "Unexpected token
'{'" errors at runtime.

Add a tsdown build step (matching sandboxed-test's pattern) and update
the exports map to point to dist/*.mjs.

Fixes #150

* fix(core): reject unbuilt source in sandbox module generator and bundle validator

Add two validation checks to prevent plugins with misconfigured exports
from silently breaking site builds:

1. generateSandboxedPluginsModule() now throws a clear error if a
   sandbox entrypoint resolves to a TypeScript/JSX source file instead
   of pre-built JavaScript. This catches the problem at site build time
   with an actionable message.

2. The `emdash bundle` command now validates that all package.json
   exports point to built files (.js/.mjs), not source (.ts/.tsx/.jsx).
   This catches the misconfiguration at plugin publish time, before
   consumers are affected.

Fixes #150

* chore: add changeset for sandbox source validation

* fix: use slash syntax for e18e rule override in oxlintrc

The test file override for e18e/prefer-static-regex used parenthesis
syntax ("e18e(prefer-static-regex)") which is the diagnostic display
format, not the config format. Changed to slash syntax to match the
top-level rule declarations so the override actually takes effect.

* test: add tests for sandbox source validation

Add tests for both validation checks:

- generateSandboxedPluginsModule: verifies it embeds pre-built JS,
  rejects .ts/.tsx/.mts source files, and includes the plugin ID in
  error messages.

- findSourceExports: verifies it flags .ts/.tsx/.mts/.cts/.jsx exports,
  accepts .mjs/.js exports, and handles conditional export maps.

Also extracts findSourceExports() from the inline bundle.ts validation
into bundle-utils.ts so it can be tested without the CLI harness.

* fix(atproto, audit-log): add build step and export built files

Same issue as webhook-notifier — both plugins exported raw TypeScript
source from their package.json sandbox exports. Add tsdown build steps
and update exports to point to dist/*.mjs.

* refactor(smoke): replace sequential per-site astro builds with recursive pnpm build

The build verification section was running `astro build` individually
and sequentially for every demo and template (~12 sites). Replace with
a single `pnpm run --recursive --filter {./demos/*} --filter
{./templates/*} build` which pnpm parallelizes automatically.

.changeset/major-boxes-juggle.md

@@ -0,0 +1,8 @@
+---
+"emdash": patch
+"@emdash-cms/plugin-webhook-notifier": patch
+"@emdash-cms/plugin-atproto": patch
+"@emdash-cms/plugin-audit-log": patch
+---
+
+Fixes sandboxed plugin entries failing when package exports point to unbuilt TypeScript source. Adds build-time and bundle-time validation to catch misconfigured plugin exports early.