Fail closed when message ls targets an unknown agent

[myobie]

Closes #91

Summary

• resolve message ls from one catalog discovery snapshot and reject identities absent from a valid or malformed catalog
• preserve the intentional flat layout only when discovery proves the selected root is catalog-less
• add explicit --orphan recovery for raw flat inbox/archive inspection and document the boundary
• cover human, JSON, count, archive, known-native, catalog-less, orphan, and malformed-catalog paths

Verification

• cargo test --test message_cli -- --test-threads=1
• cargo test --test message -- --test-threads=1
• cargo test --workspace --all-targets -- --test-threads=1
• cargo clippy --workspace --all-targets -- -D warnings
• rustfmt --edition 2024 --check src/message.rs src/main.rs tests/message_cli.rs
• cargo doc --workspace --no-deps (passes with five pre-existing rustdoc warnings outside this change)
• git diff --check
• Hosted Nix: run 30555264193, job 90913979292, success against exact head 2ccf2de1b4045797dc5d4f8f8fc8b72d76d3c28d

This remains a draft pending exact semantic review.