Fix stale install artifact cache pruning

[peters]

Summary

• route successful install/update artifact-cache cleanup through shared core helpers
• surface retained/pruned artifact counts for setup, GUI install, and update finalize logs
• add update-manager coverage for latest_full cache retention pruning stale full and delta artifacts

Fixes #181

Tests

• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --workspace --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

[Copilot]

Pull request overview

This PR addresses stale install/update artifact-cache accumulation by routing post-success artifact-cache pruning through shared surge-core helpers, surfacing prune/retain counts in key install/update paths, and adding regression coverage to ensure latest_full retention prunes stale full/delta artifacts.

Changes:

• Introduces a shared InstallArtifactCachePruneResult + prune_install_artifact_cache_dir(...) helper in surge-core, and a small helper to resolve retained-artifact sets for a resolved installer package.
• Updates update finalize, CLI setup, and installer UI install flows to use the shared prune helper and emit prune/retain counts.
• Adds an UpdateManager test ensuring latest_full retention keeps only the latest full artifact and prunes stale full/delta cache entries.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
crates/surge-installer-ui/src/install.rs	Uses shared retained-artifacts helper and reports pruning outcome after install.
crates/surge-core/src/update/manager/finalize.rs	Routes artifact-cache pruning through the shared core prune helper and logs structured counts.
crates/surge-core/src/update/manager.rs	Adds regression test verifying latest_full cache retention prunes stale artifacts.
crates/surge-core/src/installer_package.rs	Adds prune result type + directory prune helper; adds helper for deriving retained artifacts for installer cache pruning.
crates/surge-cli/src/commands/setup.rs	Uses shared retained-artifacts helper and reports pruning outcome after successful setup.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.