Fix CI: target main branch, update Go to 1.25

[Copilot]

CI never ran on PRs to main because workflows targeted master. Go versions in CI (1.21–1.23) were also incompatible with go.mod requiring 1.25.4.

• Branch trigger: master → main in ci.yml push/PR triggers
• Go version: 1.21/1.22/1.23 → 1.25 across ci.yml (test/build/lint) and release.yml
• Gitignore: Added /locus binary to prevent accidental commits

# before
on:
  push:
    branches: [ master, develop ]
  pull_request:
    branches: [ master, develop ]

# after
on:
  push:
    branches: [ main, develop ]
  pull_request:
    branches: [ main, develop ]

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