Index agent workspace pagination

[pollychen-lab]

Summary

• add a workspace_record(repo_id, workspace_id) paging index for agent workspace list
• register the new migration and down migration in the built-in migration registry
• add an EXPLAIN QUERY PLAN regression test proving workspace pagination uses the index without a temp sort or full table scan

Fixes #443.

Validation

• cargo +nightly fmt --all --check
• git diff --check
• BINDGEN_EXTRA_CLANG_ARGS='-I/usr/lib/gcc/aarch64-linux-gnu/13/include' ZSTD_SYS_USE_PKG_CONFIG=1 LIBRA_SKIP_WEB_BUILD=1 cargo test workspace_listing_hits_repo_paging_index -- --nocapture
• BINDGEN_EXTRA_CLANG_ARGS='-I/usr/lib/gcc/aarch64-linux-gnu/13/include' ZSTD_SYS_USE_PKG_CONFIG=1 LIBRA_SKIP_WEB_BUILD=1 cargo test builtin_migrations_register_current_schema_migrations -- --nocapture
• BINDGEN_EXTRA_CLANG_ARGS='-I/usr/lib/gcc/aarch64-linux-gnu/13/include' ZSTD_SYS_USE_PKG_CONFIG=1 LIBRA_SKIP_WEB_BUILD=1 cargo test builtin_runner_registers_current_builtin_migrations -- --nocapture
• BINDGEN_EXTRA_CLANG_ARGS='-I/usr/lib/gcc/aarch64-linux-gnu/13/include' ZSTD_SYS_USE_PKG_CONFIG=1 LIBRA_SKIP_WEB_BUILD=1 cargo test --test command_test agent_workspace_list_paginates_and_shows -- --nocapture

[pollychen-lab]

@genedna yeah!