Normalize pg_git SQL object qualification and add search_path regression coverage

[seanwevans]

Motivation

• Ensure SQL objects owned by the extension are referenced explicitly to avoid accidental dependence on the session search_path and to make FK/DML targets unambiguous.
• Prevent subtle bugs where unqualified names could resolve to user tables or system objects in non-default search_path environments.
• Make schema and function SQL safe to call when search_path is changed by callers or tests.

Description

• Qualified internal extension tables and references by replacing unqualified names with pg_git.<object> across core schema and function SQL, including table creation, FK constraints, DML, and queries.
• Updated schema files (sql/schema/001-core.sql, sql/schema/pgit-schema.sql) to create pg_git.* tables and to reference pg_git.* in foreign keys and inserts.
• Updated function files under sql/functions/ to use pg_git.repositories, pg_git.blobs, pg_git.trees, pg_git.commits, pg_git.refs, pg_git.remotes, and pg_git.remote_refs where appropriate, while keeping non-object identifiers (e.g., the refs column alias in get_decorated_log) unchanged.
• Added a pgtap regression test test/sql/search_path_qualification_test.sql that sets search_path to public and validates init, stage/commit, commit persistence, and HEAD/master alignment still work.

Testing

• Ran a static audit rg -nP "(?<!pg_git\.)\b(repositories|blobs|trees|commits|refs|remotes|remote_refs)\b" sql/schema sql/functions to verify unqualified object names were removed, which succeeded.
• Committed the changes (git commit) after staging, which succeeded and shows the SQL and test file were added/updated.
• Added the regression test file test/sql/search_path_qualification_test.sql to the test suite for automated execution by CI (the test file was created but the SQL test suite was not executed in this patch run).

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