fix(install): clean up stale symlinks and detect manifest spec drift

[ascarter]

Two related correctness fixes for the bin/man/completions install flow. Both surface pre-existing latent bugs that are independent of any particular asset class.

1. Reap stale symlinks before re-extracting

installFromAsset wiped the tool's ToolDir and recreated symlinks for everything in the current manifest entry — but never removed the previous install's symlinks first. If the user renamed a bin spec (e.g. bin = [\"old\"] → bin = [\"new\"]) or removed a man/completion entry, the old symlinks in BinDir/ManDir/completion dirs remained, dangling into the now-deleted ToolDir.

Fix: call m.removeToolSymlinks(name) at the top of installFromAsset, before os.RemoveAll(toolDir). The reaper only removes symlinks whose targets resolve into the tool's ToolDir, so it's safe to call when no prior install exists.

2. Detect manifest asset-spec drift in isUpToDate

isUpToDate compared only the release tag, so adding a new man/completion entry to an already-installed tool (or renaming a bin) at the same release tag was reported as "up to date" and the new symlinks were never created — the user had to know to pass `--force`.

Fix: also compare `state.Bin/Man/Completions` against the manifest entry as unordered sets, and return false when any of them differ. Adds a small `stringSlicesEqual` helper.

Tests

Two regression tests cover the behaviors:

• `TestInstallFromAssetReapsStaleSymlinks` in `internal/tool/tool_test.go`
• `TestIsUpToDateDetectsManifestSpecChange` in new `cmd/install_test.go` (plus `TestStringSlicesEqual`)

Full `go vet ./...` and `go test ./...` pass.

[ascarter]

@copilot Review this PR

[Copilot]

@copilot Review this PR

The changes look good. Here's my review:

internal/tool/tool.go — stale symlink reaping

• removeToolSymlinks is called before os.RemoveAll(toolDir), which is the correct ordering: symlinks pointing into the (soon-to-be-deleted) ToolDir are removed first, then the ToolDir itself is wiped.
• removeToolSymlinks already guards against the no-prior-install case by checking whether each link's resolved target is inside the ToolDir — safe to call unconditionally.

cmd/install.go — manifest spec drift detection

• isUpToDate now short-circuits on tag mismatch first (cheap check), then falls through to the spec comparison (slightly more expensive copy+sort). Correct and efficient.
• stringSlicesEqual treats nil and empty as equivalent, which matches how TOML unmarshals omitted fields — no false positives when a manifest omits man vs. sets it to [].
• The "up to date" console message is printed only when both the tag and spec match, so the UX is consistent with the old behaviour in the common case.

Tests

• TestInstallFromAssetReapsStaleSymlinks exercises the full round-trip: symlink created by a prior install, then InstallFromAsset called with a different bin spec — the test verifies the old link is gone and the new one exists.
• TestIsUpToDateDetectsManifestSpecChange covers tag match + spec match, tag match + spec drift (added completions), tag match + renamed bin, and tag mismatch — all four branches.
• TestStringSlicesEqual covers nil/nil, nil/empty, order-independence, mismatch, and length mismatch.

go vet ./... and go test ./... pass cleanly. No issues found.