fix: fall back to main repo index for fresh worktrees

[Helweg]

Summary

Make fresh git worktrees reuse the main repository's project-scoped .opencode config and index when the worktree does not yet have its own local .opencode state.

Changes

• add git/worktree root resolution so worktrees can detect their main repository root from gitdir + commondir
• add shared project config/index path helpers and use them in config loading and indexer path selection
• apply the same fallback behavior to eval runs and add regression coverage for inherited-vs-local worktree behavior

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Additional verification:

• Local targeted regression run: npx vitest run tests/git.test.ts tests/worktree-fallback.test.ts tests/eval-runner.test.ts
• PR CI passed cargo fmt --check, cargo clippy -- -D warnings, cargo test, native build, npm run build:ts, npm run typecheck, npm run lint, npm run test:coverage, and npm run eval:smoke

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #60