Improve hermit install error messages with remediation steps

[yurtzy]

When hermit install <pkg> fails, the output is often terse (e.g., "download failed" or "checksum mismatch") without context or next steps.

Enhancement: Include in error summaries:

• The exact URL that failed (if network)
• The expected vs actual checksum (if mismatch)
• Suggested actions: "retry with --no-verify", "check network/proxy", "update hermit index"
• Reference to docs for the specific error code

Example improvement:

❌ hermit install foo
Error: checksum mismatch for foo-1.2.3.tar.gz
Expected: abc123...
Actual:   def456...
Suggestion: The package may have been updated. Run `hermit update` and retry.

This reduces frustration and support burden for package management.