docs(skills): canonical responder-extraction command on the incident card

[ysyneu]

What

The incident reference card (skills/flashduty/reference/incident.md) did not document where an incident's responders (assignees) live in the output or how to extract them with resolved person names, so automation had to probe multiple command variants to find responders / person_name.

Changes

• Intent table: new row pointing responder lookup at detail <id> --json | jq '.responders[]'.
• Gotchas: one bullet documenting the canonical extraction pipeline, the responder object schema (person_id, person_name, email, assigned_at, acknowledged_at, as), the get top-level-array difference, and the two projection traps (list default compact projection omits responders; detail --fields only prints named fields).

Verification (code-traced, no live creds)

• internal/cli/incident.go: get prints result.Items (top-level array) in structured mode; detail prints a single IncidentInfo; plain-text paths print a Responders: name1, name2 line.
• go-flashduty v0.14.2 models_gen.go: IncidentInfo.Responders []Responder; Responder carries person_id / person_name / email / assigned_at / acknowledged_at / as, with person_name and email filled by the server.
• make check-cards → skilldoc: cards OK; go test ./internal/skilldoc/ → ok.