fix: preserve explicit notification preference false

[ysyneu]

What

assigned_to.notify.follow_preference / notify.follow_preference is a tri-state field:

• omitted — use each responder's personal notification preference;
• true — explicitly use the personal preference;
• false — use the personal_channels listed in the request.

The OpenAPI schema modelled it as an optional non-nullable boolean, so the generator emitted
FollowPreference bool with omitempty. Go's encoder drops false, so the SDK could never send
the one value that activates personal_channels — a request carrying personal_channels was
accepted but delivered over the responder's personal preference instead.

Change

Model the field as "type": ["boolean", "null"] in the OpenAPI source (already merged upstream in
flashduty-docs) and re-sync + regenerate. The generator's existing nullable-request-scalar rule
turns it into a pointer:

FollowPreference *bool `json:"follow_preference,omitempty"`

Callers send an explicit false with flashduty.Bool(false); a nil pointer stays off the wire.
The same contract backs incident creation, responder addition and incident assignment, so all three
request schemas are corrected and AssignedTo now exposes notify.

.gitignore additionally covers local-only scratch directories so they cannot be committed by accident.

Compatibility

FollowPreference changes from bool to *bool on the three request-side notify structs. Keyed
literals that set the field must switch to flashduty.Bool(true) / flashduty.Bool(false).
Response-side structs (ScheduleNotifyBy, EscalateTargetBy, FeedDetailIncidentAssignNotify) and
the escalation-rule / schedule request structs are unchanged — their backend counterparts are plain
booleans where omitted is equivalent to false.

Validation

• make check — gofmt/gci clean, golangci-lint 0 issues, go test -race ./... green, build OK.
• New generator test: an optional nullable request boolean emits *bool with omitempty.
• New wire tests over /incident/create, /incident/responder/add and /incident/assign:
  Bool(false) serialises as "follow_preference":false; nil omits the key entirely.

Replaces #56 (same code, rebased onto a clean history).