Add fixedMajorVersion option for multiple major version support

[Konboi]

Summary

Add tagpr.fixedMajorVersion configuration option that allows tagpr to target a specific major version when creating releases. This enables maintaining multiple major versions on different branches.

Closes #248

Use case:

• v1 branch for v1.x.x releases
• main branch for v2.x.x releases

Changes

• Add tagpr.fixedMajorVersion config option (also via TAGPR_FIXED_MAJOR_VERSION env)
• Accept both numeric ("1") and v-prefixed ("v1") formats
• Filter tags in latestSemverTag() to only consider matching major versions
• Validation error on invalid values during config reload

Example Usage

[tagpr]
    releaseBranch = v1
    fixedMajorVersion = 1

Or via environment variable:

TAGPR_FIXED_MAJOR_VERSION=1

Discussion Points

• Currently only supports major version fixing. Minor version fixing (e.g., for LTS support like v1.0.x) is not implemented as the use case seems rare. Open to discussion if needed.
• If we want to support minor version fixing in the future, we could rename and generalize to fixedVersion with pattern support:
  • fixedVersion = "1" → fix major version (v1.x.x)
  • fixedVersion = "1.2" → fix major and minor version (v1.2.x)

🤖 Generated with Claude Code

[Konboi]

@Songmu I've fixed conflict. Could you review when you have time?
Thank you!