feat(librarian): support active processing adapters

[AlainDor]

Summary

• Add Claude/Codex/Copilot librarian adapter selection and CLI overrides.
• Teach the harness to default processing from the active wrapped agent or explicit config, and record active_agent/selected_adapter in capture summaries.
• Document adapter config and add coverage for Codex, explicit config, and Copilot.

Verification

• cargo build --workspace
• cargo test --workspace
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings

Closes #19