[V2:10] Add Prepared Runtime Build Pipeline

[ritorhymes]

Part of V2: Multi-repo local build system.

Summary

This PR moves the prepared runtime build pipeline out of main.rs and into pipeline.rs, where it consumes ResolvedExecution from V2:08 and the workspace-local theme model from V2:09. Prepared::prepare now owns the full preparation sequence (disposable repo setup, source and sibling materialization, changed-file linting via eipw, markdown preprocessing, local theme materialization into repo/themes/eips-theme, and Zola build/check), while main.rs is reduced to dispatch and locking. The move preserves build/check behavior aside from two small targeted fixes (sibling-merge homepage preservation and force-refresh of prepared scratch refs), so review should focus on the new ownership boundary, theme materialization, and verifying those fixes.

• Add pipeline.rs to own the Prepared runtime pipeline.
• Move source preparation, eipw linting, markdown preprocessing, and Zola build/check calls out of main.rs.
• Consume ResolvedExecution to prepare the disposable build repo from resolved source, theme, build-root, and base-URL policy.
• Materialize the workspace-local theme into repo/themes/eips-theme before Zola runs.
• Keep main.rs focused on command dispatch, build-root creation, locking, and runtime operation selection.
• Add a pipeline test for materializing the workspace-local theme into the mounted Zola theme path.
• Preserve the active repo homepage during sibling merges and force-refresh prepared git scratch refs so rebased local branches are not built from stale commits.

Review Notes

This PR is the prepared runtime ownership split. V2:08 resolves execution policy, V2:09 requires the workspace-local theme, and this PR centralizes the actual build/check preparation around those resolved inputs.

Prepared::prepare now owns the runtime preparation sequence: initialize the disposable build repo, clone/fetch the active source repo, collect changed proposal files for linting, merge sibling proposal sources, run eipw lint, preprocess markdown, and materialize the local theme for Zola.

The theme handling changes shape here. V2:09 made Zola use a workspace-local theme; this PR materializes that local theme into the prepared repo under repo/themes/eips-theme before calling Zola, so Zola sees the normal mounted theme layout.

This PR does not add new CLI surface. It preserves the existing build/check runtime behavior while moving the implementation out of main.rs.

The existing minimal Prepared::serve method moves with Prepared so the type remains coherent, but this PR does not add the dirty watcher, theme sync, server binding, or final serve behavior. Those land in V2:11.

Verification

• Review src/pipeline.rs for Prepared::prepare, source materialization, changed-file lint selection, sibling merge, markdown preprocessing, theme materialization, and build/check calls.
• Review src/main.rs for the reduced dispatch-only runtime flow.
• Review src/find_root.rs for the direct crate::layout::CONTENT_DIR import now that main.rs no longer brings CONTENT_DIR into scope.
• Review the pipeline test for materializing a workspace-local theme into repo/themes/eips-theme.
• Confirm that serve watcher/sync behavior is not introduced here and remains scoped to V2:11.