refactor: split workspace modules

[oso95]

Summary

• split the large workspace polling and gpui modules into smaller focused child modules
• keep the root workspace modules as orchestration entry points and move ownership-aligned tests with the extracted modules
• replace temporary split-plan docs with permanent workspace architecture docs under docs/architecture/workspace/

Verification

• cargo clean
• cargo fmt --all -- --check
• cargo check --workspace --all-targets --all-features
• cargo build --workspace --all-features
• cargo test --all --all-targets --all-features
• cargo test -p codirigent-ui --lib --features gpui-full
• cargo clippy --all --all-targets --all-features -- -D warnings
• cargo check -p codirigent-ui --features gpui-full
• git diff --check

Notes

• full verification was run on macOS in this workspace
• Windows verification is still expected separately before merge