refactor(cli): split install selection and manifest resolution helpers

[peters]

What changed

• convert the surge install command implementation from a single install.rs file into an install/ module tree
• move install target and channel selection logic into crates/surge-cli/src/commands/install/selection.rs
• move install manifest and storage-resolution helpers into crates/surge-cli/src/commands/install/resolution.rs
• keep execute and the local/remote install execution flow in install/mod.rs so this phase only extracts early-resolution logic
• update the maintainability baseline entry from install.rs to install/mod.rs with the reduced production-line count

Why

crates/surge-cli/src/commands/install.rs had become the largest file in the workspace and mixed interactive selection, manifest/storage resolution, and install execution in one module. This phase peels off the low-risk helper surfaces first so later install refactors can work against a smaller orchestration root.

Impact

• no intended CLI behavior change
• existing install tests continue to target the same helper behavior, but now through focused internal modules
• the maintainability guardrail now tracks the new install/mod.rs path instead of the removed flat file

Validation

• git submodule update --init --recursive
• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• ./scripts/check-maintainability.sh
• cargo test -p surge-cli commands::install::tests::
• cargo check -p surge-cli
• 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