Fix internal state path for multiple worktrees

[wantaekchoi]

Summary

This changes git-crypt internal state resolution to use git rev-parse --git-common-dir instead of --git-dir, so linked worktrees share the same key
state.

The regression test coverage and CI wiring are split into separate commits.

Why

With linked worktrees, storing git-crypt state under the worktree-local git dir can make key state invisible to other worktrees in the same repository.
Using the common git dir keeps that state shared as intended.

Validation

• make test passed in a Rocky Linux 8.10 Docker environment
• verified that the worktree test checks for key state under git-common-dir/git-crypt/keys/default
• verified that the worktree-local git-dir/git-crypt/keys/default path is not used

[alerque]

Please split the test, CI, and actual fix parts into different commits and properly label them. This PR is doing a lot more than what it says on the tin.

Also please declare what if any LLM is being used here.