refactor(cli): split remote install helpers

[peters]

Summary

• split crates/surge-cli/src/commands/install/remote.rs into focused leaf modules for execution, published installer planning, activation, staging, state, and shared types
• keep the runtime install::remote surface stable while moving test-only helpers behind #[cfg(test)] reexports
• remove the stale remote baseline entry and update the cleanup rollout tracker for the active phase

Validation

• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• dotnet format dotnet/Surge.slnx --verify-no-changes
• dotnet test dotnet/Surge.slnx --configuration Release
• cargo test -p surge-cli commands::install::
• cargo clippy -p surge-cli --all-targets --all-features -- -D warnings -W clippy::pedantic
• ./scripts/check-maintainability.sh

Impact

• no intended user-facing behavior changes
• the remaining maintainability backlog is reduced to crates/surge-cli/src/commands/install/mod.rs
• the active cleanup phase now matches the codebase and baseline docs