Extract the ClickPipes runtime and finalize the cloud CLI split

[sdairs]

Parent: #318

Stack position: 6 of 6. Completes the cloud CLI reorganization.

Goal

Move the ClickPipes runtime into its domain module, retire the cloud monoliths, and document the final command layout.

Scope

• Move ClickPipes handlers, request builders, parsers, output, CloudClient wrapper methods, and unit tests into the ClickPipes module.
• Remove cloud/commands.rs once all domain code has moved.
• Reduce cloud/client.rs to core client/auth/error/shared functionality.
• Reduce cloud/cli.rs to top-level cloud arguments/commands, domain re-exports, and top-level exhaustive classification.
• Keep the cross-domain subprocess request-shape test file intact; splitting that test file is separate work.
• Update AGENTS.md command-adding guidance for co-located per-domain clap, handlers, builders, wrappers, and tests.

Acceptance

• No ClickPipes runtime code remains in a generic commands module.
• commands.rs is removed and client.rs/cli.rs have only their final shared responsibilities.
• ClickPipes request shapes, file/base64 handling, output, and error behavior are unchanged.
• All clap, builder, inline, subprocess wiremock, and workspace tests pass.
• cargo clippy --workspace --all-targets -- -D warnings and cargo fmt --all --check pass.
• Contributor documentation points new cloud commands to their domain module.

Out of scope

• Splitting tests/cli_request_shape_test.rs.
• Changing Postgres direct-client behavior, cloud auth, commands, flags, or outputs.