#17636: Build: add runtime-only build target

## Summary - Adds `pnpm build:runtime` for packagers/runtime builds. - `pnpm build` remains unchanged (still generates plugin-sdk d.ts for TS consumers). ## Why Packagers that build from source in clean environments often do not need plugin SDK declaration output for runtime packaging, but the declaration build can be expensive. ## Tests - `pnpm -s build:runtime` <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds a `pnpm build:runtime` script that runs the full build pipeline minus the plugin-sdk TypeScript declaration generation (`tsc` + entry `.d.ts` stub writing). This lets packagers building from source in clean environments skip the expensive declaration emit when only runtime JS output is needed. The existing `pnpm build` remains unchanged. - Adds `build:runtime` script in `package.json` that mirrors `build` but omits `build:plugin-sdk:dts` and `write-plugin-sdk-entry-dts.ts` - Adds corresponding changelog entry under the unreleased section - No runtime-critical build steps are missing from the new target — all skipped steps produce only `.d.ts` files <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it adds a new build target without modifying existing build behavior. - The change is minimal and additive: a single new npm script that is a strict subset of the existing `build` script, plus a changelog entry. No existing scripts, runtime code, or build outputs are modified. The skipped steps produce only `.d.ts` declaration files that have no runtime impact. The existing `build` and `prepack` scripts remain unchanged. - No files require special attention. _{Last reviewed commit: 3254bae} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->