v3: yield the version flag's -v alias to a user-defined flag

[c-tonneslan]

Fixes #2229.

The default version flag carries `-v` as an alias. A user-defined flag that also wants `-v` (canonical case: `--verbose / -v`) wasn't actually broken at parse time: `-v` ended up setting the user flag. But `checkVersion` went through `cmd.Bool`, which resolves aliases, so it asked for the value of `"v"` and got back the user flag's value. The end result: passing `-v` was silently treated as "print version and exit."

Two narrow changes:

• During root setup, drop any aliases from the local copy of the version flag that are already claimed by a user-defined flag's name or alias. Keeps the user flag the sole owner of the short form.
• `checkVersion` now finds the actual version flag attached to the command (matching against the canonical primary name from the global VersionFlag) and checks `IsSet` directly, so it can't be fooled by a same-named alias on a different flag.

Added `TestVersionFlagYieldsAliasToUserFlag` covering the reporter's reproducer.

[dearchap]

@c-tonneslan Can you rebase off main and fix test cov ?

[dearchap]

Review Summary

1. dropClashingAliases in command_setup.go — ✅ Merge with fix

Good improvement over the existing flagNamesInUse approach. Instead of dropping the entire --version flag when -v clashes, it only removes the clashing -v alias, preserving --version.

Minor issue: dropClashingAliases is called with cmd.Flags but flagNamesInUse just below uses cmd.allFlags() (which also includes MutuallyExclusiveFlags). If a mutually exclusive flag group claims -v, dropClashingAliases won't see it. Pass cmd.allFlags() instead of cmd.Flags for consistency.

2. checkVersion rewrite in help.go — ❌ Drop this change

Keep the current cmd.versionFlag.IsSet() implementation. The proposed name-based matching (names[0] == primary) is fragile — it could match a user-defined flag whose primary name happens to be "version", causing --version to trigger ShowVersion based on the wrong flag's state.

3. New test — ⚠️ Redundant; consider expanding

The existing TestCommand_Run_CustomFlagCanUseVersionAlias already covers the same scenario. Consider testing that --version still works after -v is yielded to a user flag — neither test currently covers that.

[c-tonneslan]

Rebased on main, conflict is gone.

Addressed the review:

• dropClashingAliases now takes cmd.allFlags() instead of cmd.Flags, so a mutually-exclusive group claiming -v is also seen.
• Dropped the checkVersion rewrite. Once the clashing alias is stripped in setup, cmd.versionFlag.IsSet() already does the right thing, so help.go is untouched now.
• Replaced my redundant test (the existing TestCommand_Run_CustomFlagCanUseVersionAlias already covers -v) with one that checks --version still prints after -v is yielded. That case fails on main today. Added a small table test for dropClashingAliases to cover the helper.