[codeforester]

Fixes #579

Summary

• add base_git_worktree_path_is_valid as the shared linked-worktree validity predicate
• distinguish valid linked and detached worktrees from ordinary repositories, missing paths, and stale Git worktree metadata
• add focused BATS coverage for spaced paths, detached worktrees, ordinary clones, stale metadata, and diagnostics
• document and publish the API through the manifest, generated reference, v2 contract, and symbol map
• make the release-tree compatibility check validate the historical manifest so additive stable v2 APIs do not have to exist in the v2.0.0 tree

Issue

Fixes #579; follows the design-level follow-up from basefoundry/base#2621.

Validation

• bats lib/bash/git/tests/lib_git.bats
• ./tests/validate.sh
• tests/lint-warnings.sh
• scripts/api-manifest check
• git diff --check

API Impact

Adds the stable base_git_worktree_path_is_valid <path> Git helper. It returns 0 only for a valid linked worktree, 1 for ordinary, missing, or stale paths, and 2 for usage errors. It is read-only and suppresses Git diagnostics for false predicates.

Demo Impact: None.

AI context: unchanged; reusable library contract only.