refactor: remove legacy protocol scaffolding

[zatevakhin]

changes

• remove production-dead legacy ClientMsg/SessionScope outbound types, protocol PromptBlock, associated helpers, and serializer tests
• remove obsolete session-page/fork/file-index/event-envelope/AgentEvent/mesh-invite-list and other orphan protocol DTOs
• preserve active command prompts, ACP session adapters, undo/redo, EventKind payloads, mesh/remote, and auth/oauth DTOs with their active tests
• retain the audit-event contract in the local app regression fixture: { "timestamp": 123, "kind": { "type": "...", "data": { ... } } }, using kind: EventKind and timestamp: Option<i64>
• restore the base crate-wide #![allow(dead_code)]; no local dead_code/cfg_attr allowances are introduced by this PR

validation

• cargo fmt --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• cargo test --all-features (734 passed)
• searches confirm deleted legacy protocol definitions are absent, the app fixture is nested, src/lib.rs matches the base, and the final base diff touches only src/app.rs and src/protocol.rs

deferred

Crate-wide dead-code cleanup and the broader event/UI/session migration are intentionally deferred, including protocol splitting and EventKind redesign.