refactor(cli): split install root orchestration

[peters]

Summary

• extract the local install workflow plus shared runtime/progress helpers out of crates/surge-cli/src/commands/install/mod.rs
• move the high-level tailscale install decision tree into install/remote/mod.rs so the root stays orchestration-focused
• remove the final maintainability baseline entry and update the cleanup tracker for the last file split

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 check -p surge-cli
• 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
• docs/architecture/maintainability-baseline.txt is now empty on this branch
• no second-wave oversized Rust file splits remain once this PR lands