Add declared native delivery selector

[myobie]

Summary

• parse deliver "mcp" and deliver "app-server" as typed native delivery selectors
• reject unknown, duplicate, mixed, malformed, and explicit-null delivery declarations
• ensure deliver never lowers to the legacy screen DING task
• include delivery in catalog semantic diffs
• make Doctor report valid running agents that declare no transport without failing them
• keep invalid transport values as hard catalog problems

Validation

• cargo check --all-targets --locked
• cargo test -p agent-spec --locked
• cargo test --lib --locked
• cargo test --test doctor --locked
• cargo test --test validate --locked
• cargo test --test reconcile --locked
• cargo clippy --all-targets --all-features --locked completes with only pre-existing current-main warnings
• cargo test --test catalog_diff --locked: 15 pass, including the new delivery proof; one pre-existing current-main nested non-agent KDL test fails
• Nix is not installed on this host; hosted nix flake check is the authoritative gate

The VRS contract is in draft PR #229 and must be reviewed first. This branch is based directly on current main, not stacked on the documentation branch. No merge is requested before Nathan reviews the exact head.

[myobie]

Folded into the reviewable implementation replacement #241 at exact head d8269638bc6a82e4b2f9ce29b2e8509be5bd2337. The replacement preserves this draft’s relevant history and has exact-head hosted Nix green. Closing this superseded draft; its branch is intentionally retained.