Fall back to a resolved @get-bb/plugin-sdk entry for the legacy SDK alias in source dev mode

[technicalpickles]

What was wrong

pluginSdkAlias in apps/server/src/services/plugins/plugin-runtime.ts only registered the legacy @bb/plugin-sdk specifier alias (kept since the #1574 rename to @get-bb/plugin-sdk) when a prebuilt dist/plugin-sdk-runtime.js bundle existed next to the running module. That bundle is produced only by the server's build script. pnpm dev and pnpm dev:desktop run the server from src/ via tsx and never run build, so pluginSdkAlias silently resolved to undefined in every source dev workflow. Any installed plugin still importing the pre-rename @bb/plugin-sdk specifier (e.g. bb-plugin-dispatch@0.1.3) then failed to load with Cannot find module '@bb/plugin-sdk', even though the same plugin loads fine in a packaged/production build (nightly, npx bb-app). See #2334 for the full repro and log evidence.

What changed

• apps/server/src/services/plugins/plugin-runtime.ts: extracted resolvePluginSdkAliasTarget(). When the prebuilt runtime bundle is missing, it falls back to import.meta.resolve("@get-bb/plugin-sdk") (the package already resolves naturally in source dev mode per the existing doc comment) and aliases the legacy specifier to that resolved path instead of leaving it unaliased.
• apps/server/test/services/plugins/plugin-sdk-alias.test.ts: added a test asserting the fallback resolves to the plugin-sdk source entry when no prebuilt bundle is present (this is how the suite always runs, so it exercises the exact dev-mode gap).

No wire/protocol changes, no CLI/doc surface changes — this only affects how the server resolves an internal module alias during plugin loading.

How you verified

• pnpm exec turbo run typecheck --filter=@bb/server — passes.
• pnpm exec turbo run test --filter=@bb/server -- --run test/services/plugins/plugin-sdk-alias.test.ts — both tests pass (the new one fails without the fix, since resolvePluginSdkAliasTarget() would return undefined).
• Manual repro: ran pnpm dev:desktop before the fix and confirmed bb-plugin-dispatch@0.1.3 failed with Cannot find module '@bb/plugin-sdk' in the server log and showed "Failed" in the Extensions UI. Applied the fix, restarted pnpm dev:desktop, confirmed the same plugin now loads (plugin dispatch@0.1.3 loaded in the log) and shows healthy/enabled in the Extensions UI with no error banner.
• Verified against a clean install in a fresh worktree off main (not just the branch where I found the bug), so this isn't an artifact of leftover state.

Fixes #2334

AGENT GENERATED