[V2:05] Add Source Materialization Layer

[ritorhymes]

Part of V2: Multi-repo local build system.

Summary

This PR adds the source-materialization boundary that later workspace, runtime, and serve/watch PRs use to choose between clean cloned sources and dirty local working trees. It introduces SourceMaterialization::{Clean, Dirty} plus git helpers for availability, clone/fetch, dirty working-tree copying, tracked-path sync, and sibling merge behavior. Existing build and changed-file flows still use Clean, so review should focus on the git.rs source-state contract, not on a switch to dirty as the default.

• Add SourceMaterialization::{Clean, Dirty} and use manifest-derived RepositoryUse values to carry resolved repository endpoints into git::Fresh::new.
• Add repository availability, clone, fetch, dirty status, and working-tree materialization helpers in git.rs.
• Add tracked working-tree path discovery and sync helpers for dirty local source materialization.
• Update sibling merge behavior so local file sibling sources follow checked-out HEAD, while remote sibling sources continue fetching master.
• Route existing build and changed-file flows through clean source materialization.

Review Notes

This is the git/source foundation for later workspace and runtime behavior. It does not make dirty local workspaces the default yet; existing build and changed flows continue using clean cloned sources.

Workspace init uses the clone/repo availability pieces in V2:06. Execution policy starts selecting clean versus dirty source modes in V2:08. The prepared runtime pipeline consumes materialized active and sibling sources in V2:10, and serve/watch behavior consumes the dirty sync helpers in V2:11.

Review this as the source-materialization boundary: git.rs owns repository availability, cloned source setup, dirty working-tree copying, tracked path sync, and sibling merge behavior. Later PRs decide when those paths are used.

Dirty materialization rejects conflicted index entries before copying and fails explicitly on unsupported dirty paths, so later runtime PRs can fail with an explicit source state error instead of silently building from an ambiguous tree.

RepositoryUse carries the active Build.toml location and declared sibling repository endpoints into source materialization, keeping repository resolution explicit at the git boundary.

Verification

• Review src/git.rs for SourceMaterialization, RepositoryUse, clone/fetch setup, dirty materialization, tracked path sync, dirty rejection errors, and sibling merge behavior.
• Review src/changed.rs and src/main.rs for the clean materialization wiring used by existing behavior.
• Review src/config.rs for manifest-derived repository locations and RepositoryUse construction used by source materialization.
• Review git materialization tests for tracked file copying, staged file sync, symlink handling, and non-repository errors.