fix(pgpm): clear update cache after pgpm update and use semver comparison

[pyramation]

Summary

Two small fixes for the pgpm update check:

1. Stale cache after update: After pgpm update succeeds, the update-check cache (~/.pgpm/cache/update-check.json) was not cleared, so subsequent commands kept showing "update available" until the 24h TTL expired. Now we delete the cache file after a successful install using appstash (already a dependency) to resolve the cache path.

2. String-based version comparison: The upstream checkForUpdates in @inquirerer/utils uses JS string comparison (>) which silently fails for multi-digit semver segments (e.g., "3.10.0" > "3.9.0" is false). Instead of trusting updateResult.hasUpdate, we now use semver.gt() on the returned latestVersion directly. semver is already a dependency of pgpm/cli.

This is a minimal alternative to #704, which added execFile calls (the CLI invoking itself), new exported functions, and 222 lines of tests. This PR achieves the same result in ~13 lines across 2 files with no new dependencies or exports.

Review & Testing Checklist for Human

• Verify the cache path appstash('pgpm').cache + '/update-check.json' matches what @inquirerer/utils checkForUpdates writes to (it should — both call appstash('pgpm'))
• End-to-end: run pgpm update, then immediately run another pgpm command (e.g. pgpm --help) and confirm no stale "update available" prompt appears
• Verify the semver default import works correctly with the existing "semver": "^7.6.2" dependency and @types/semver in devDependencies

Notes

• The upstream string comparison bug still lives in @inquirerer/utils checkForUpdates — this PR works around it at the call site. A proper fix upstream would let us revert the semver workaround in commands.ts.
• Requested by: @pyramation
• Link to Devin run

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