refactor: separate mesh domain values

[zatevakhin]

What changed

• Extracts application-facing mesh values into domain::mesh without serde derives.
• Renames mesh transport values to explicit *Dto types and converts them at the ACP boundary.
• Preserves opaque remote-attach config_options and snapshot values unchanged.

Behavior

This is a pure boundary extraction: mesh status/node/session/invite behavior, extension calls, request payloads, cursor handling, current-cwd load behavior, and detached attached=false refresh behavior are unchanged.

Deferred To PR6

• Any behavioral interpretation of attached=false.
• Remote cwd selection changes.
• Applying attach config options or hydrating attach snapshots.
• Reconnect semantics and related mesh behavior changes.

Validation

• git diff --check
• 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 (739 passed)