refactor: remove legacy protocol scaffolding

[zatevakhin]

changes

• remove the production-dead legacy ClientMsg/SessionScope outbound surface, protocol PromptBlock, helper implementation, and serialization tests
• remove obsolete session page, fork result, event envelope, agent event wrapper, file index, mesh invite list, and other orphan protocol DTOs
• preserve active command prompts, native ACP session adapters, auth/oauth, undo/redo, event payload, mesh, and remote DTOs
• replace app event serde coverage with a local test-only flattened TestAgentEvent fixture
• remove the crate-wide dead_code allowance, delete bounded dead items exposed by the compiler, and retain only narrow justified item-level allowances plus the generated themes allowance

validation

• cargo fmt --all -- --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)
• acceptance searches confirm deleted names and protocol PromptBlock are absent; command PromptBlock and active DTO references remain

deferred

protocol module splitting, EventKind redesign, and active DTO redesign remain deferred to pr8.