fix: sandbox-friendly git tests and linked-gitdir support

[simtel12]

Summary

Some environments (including constrained sandboxes) block writes under a worktree’s .git directory while still allowing a sibling git directory. Repositories created with git init --separate-git-dir also keep metadata outside <worktree>/.git. This pull request resolves the real Git directory via git rev-parse, uses it for rebase detection and pgit-sync-state.json, and updates integration tests to use separate git dirs under target/ with an optional PGIT_TEST_TMP override.

Changes

• Repo::git_directory(): Resolves the absolute git directory using git rev-parse --absolute-git-dir, with fallback to --git-dir and path normalization relative to the worktree when Git returns a relative path.
• Rebase and sync state: is_rebase_in_progress, read_sync_state, and write_sync_state read and write under the resolved git directory instead of assuming .git/ lives directly inside the worktree.
• Integration tests (tests/test_git_ops.rs): Ephemeral repos use git init --separate-git-dir; scratch roots default to target/pgit-integration-tmp/ (override with PGIT_TEST_TMP); commit/tag GPG signing is disabled in-repo so tests do not depend on ~/.gnupg; teardown removes both the worktree and the companion git directory.
• AGENTS.md: Documents the usual cargo build, cargo test, Clippy, and rustfmt --check workflow for contributors.

Test plan

• cargo test (library unit tests and test_git_ops integration tests)