fix: harden in-place update command

[EvanProgramming]

Fixes #160

What changed

• invoke the resolved uv executable for both normal and uncached update attempts
• report stdout and stderr diagnostics, including failures from the retry attempt
• show a clear restart instruction after the pinned immutable release update succeeds
• update all README command tables and add a stdout-only failure regression test

Validation

• disposable Python 3.12 environment: focused /update tests (3 passed)
• exact pinned GitHub v2.0.3 wheel install smoke in a disposable uv tool directory
• make lint
• git diff --check