perf(build): deduplicate plugin-sdk chunks to fix ~2x memory regression

Bundle all plugin-sdk entries in a single tsdown build pass instead of
38 separate builds. The separate builds prevented the bundler from
sharing common chunks, causing massive duplication (e.g. 20 copies of
query-expansion, 14 copies of fetch, 11 copies of logger).

Measured impact:
- dist/ size: 190MB → 64MB (-66%)
- plugin-sdk/ size: 142MB → 16MB (-89%)
- JS files: 1,395 → 789 (-43%)
- 5MB+ files: 27 → 7 (-74%)
- Plugin-SDK heap cost: +1,309MB → +63MB (-95%)
- Total heap (all chunks loaded): 1,926MB → 711MB (-63%)

tsdown.config.ts

@@ -116,12 +116,12 @@ export default defineConfig([
       "line/template-messages": "src/line/template-messages.ts",
     },
   }),
-  ...pluginSdkEntrypoints.map((entry) =>
-    nodeBuildConfig({
-      entry: `src/plugin-sdk/${entry}.ts`,
-      outDir: "dist/plugin-sdk",
-    }),
-  ),
+  nodeBuildConfig({
+    // Bundle all plugin-sdk entries in a single build so the bundler can share
+    // common chunks instead of duplicating them per entry (~712MB heap saved).
+    entry: Object.fromEntries(pluginSdkEntrypoints.map((e) => [e, `src/plugin-sdk/${e}.ts`])),
+    outDir: "dist/plugin-sdk",
+  }),
   nodeBuildConfig({
     entry: "src/extensionAPI.ts",
   }),