feat: add prune command to remove superseded versions

[tr1v3r]

Motivation

Each installed Go version takes up a few hundred megabytes. Upgrading within a minor series (e.g. g install 1.24 resolving to the latest 1.24.x) leaves the older patch versions installed, and today they can only be reclaimed by calling g uninstall once per version.

This adds a g prune command that removes superseded versions automatically.

Behavior

• Groups installed versions by major.minor series and keeps only the newest version of each series (e.g. with 1.19.10, 1.19.13, 1.20.5, 1.24.5, 1.24.6 installed, 1.19.10 and 1.24.5 are removed).
• Never removes the version currently in use, even if it is not the newest in its series. (This protection is also missing from g uninstall today — worth a follow-up, but kept out of this PR to stay minimal.)
• Prereleases are superseded by the final release of the same series (1.25rc1 is pruned once 1.25.0 is installed).
• --dry-run prints what would be removed without deleting anything.
• Idempotent: a second run reports Nothing to remove.

$ g ls
  1.19.10
  1.19.13
  1.20.4
* 1.24.3
  1.24.6
$ g prune --dry-run
Would remove 1.19.10
$ g prune
Removed 1.19.10

Implementation

• cli/prune.go: thin action over listLocalVersions + a pure superseded() helper (versions are sorted ascending, so a version is superseded when its successor belongs to the same minor series).
• version/version.go: adds Major()/Minor() accessors for series grouping.
• cli/commands.go: registers the prune command.
• Bilingual README updates (README.md, README_CN.md).
• cli/prune_test.go: table-driven tests covering grouping, in-use protection, prerelease ordering, and major-series independence.

Testing

• go build ./..., go vet ./cli/ ./version/, gofmt clean.
• go test ./cli/ ./version/ pass.
• Packages collector, pkg/http, pkg/sdk/github currently fail in my environment on both this branch and pristine master (they hit real network URLs behind a corporate proxy), so they are unrelated to this change.
• Manual smoke test against an isolated G_HOME (create version dirs, symlink go to an older patch, run g prune / g prune --dry-run / g ls).