refactor(ci): simplify release workflow to dispatch-only

[kolkov]

Summary

Simplifies the release workflow by removing release branch support and keeping only workflow_dispatch trigger.

Problem

The previous workflow supported two release methods:

1. workflow_dispatch - manual trigger with version input
2. push to release/* branch - auto-trigger

The release branch approach had a critical flaw: after creating the release, the workflow tried to merge the release branch back to main, which failed due to branch protection rules. GitHub Actions with GITHUB_TOKEN cannot bypass protected branch rules.

Solution

Remove the release branch trigger entirely. Use only workflow_dispatch:

# New release process:
gh workflow run release.yml -f version=0.9.4

Changes

• Remove push trigger on release/* branches
• Remove "Merge release branch to main" step (doesn't work with protected branches)
• Simplify version determination logic (no branch name parsing)
• Update documentation comments

Benefits

• Simpler workflow (one trigger instead of two)
• No merge-back issues with protected branches
• Tag created only after tests pass
• Standard approach used by GoReleaser and other Go projects

References

• Creating a tag and releasing from a GitHub Action
• GitHub Discussion: Allowing github-actions to push to protected branch