chore: unify build/CI tooling with zx script, add version utils & tests ✨

**✨ Features**
- Add a unified zx-based build script (scripts/build.mjs) to run build, dev, ci, package, test, validate, upgrade, and version commands from a single entrypoint.
- Expose many npm script aliases that delegate to the zx build script to simplify developer workflows.

**🐛 Fixes**
- Include vscode.LanguageModelThinkingPart handling in LiteLLMChatProvider so thinking-part tokens are accumulated and total output tokens are counted correctly.

**🧹 Chores**
- Replace separate CI steps with a single GitHub Actions step that runs zx scripts/build.mjs ci and set VSCODE_TEST_RESULTS_DIR for test artifacts.
- Update docker-build workflow triggers to include package.json, package-lock.json, and scripts/build.mjs.
- Bump package version to 1.6.0-rc3 and consolidate/replace many npm scripts with zx-based commands.
- Upgrade dev deps (typescript -> ^6.0.3) and add zx (^8.8.5), and adjust posthog sourcemap packaging to use the new build flow.

**🧪 Tests**
- Add version utilities (scripts/versionUtils.mjs) and unit tests (scripts/test/build-version.test.mjs) to validate version parsing/bumping logic.