fix: preserve explicit notification preference false

[pijiang3]

Release change

Preserve explicit follow_preference: false in incident notification override requests so personal_channels is honored.

Compatibility

This changes generated FollowPreference fields from bool to *bool. Existing keyed literals must use flashduty.Bool(false) or flashduty.Bool(true). Publish this change as v0.13.0.

Validation

• GitHub Actions: Go 1.24 and stable build/test passed.
• Lint passed with 0 issues.
• Generate Drift passed.
• Explicit-false and omitted-field wire tests pass.
• Independent branch review found no blocking findings.

[ysyneu]

Superseded by #57, which carries the identical code and OpenAPI change on a clean history. The design-note file under docs/ was local-only scratch and should not have been committed; .gitignore now covers that path. Thanks @pijiang3 — your commit is preserved verbatim in #57.