fix(skill): resolve person_id via person infos, not member-list pagination

[ysyneu]

What

Schedule/oncall output returns person_ids, not names. This points the agent at the existing batch resolver fduty person infos <person_id> … (POST /person/infos) for that lookup, and removes the guidance that sent it down the wrong path.

• schedule.md — replace the "join fduty member list on member_id" flow with a one-shot fduty person infos call; add a person_id ≠ member_id warning.
• member.md — cross-reference the person group from member (the agent looked under member, never found person infos); add a person_id resolution gotcha.
• oncall.go — correct the oncall who Long help, which falsely claimed "member list has no by-id lookup" and recommended dumping the roster (the command's table output already resolves names inline).

Root cause

The batch resolver already exists; this was a discoverability + namespace-confusion bug, not a missing endpoint. person_id (returned by schedule/oncall/incident/alert) is a different namespace from member_id (returned by member list). The schedule.md card and the oncall who help told agents to resolve names by paginating fduty member list and matching member_id == <person_id> — wrong join key, and a 20+ page full-roster scan that silently drops people on later pages.

Evidence (run audit-2026-06-26)

• sess_8fKpUejeyKfHt2hx5XNSsc (steps 14–46): agent paginated member list pages 1–22 matching member_id against schedule person_ids, tried a non-existent fduty member get --id, and missed a person who lived on page 21.
• sess_UtK6eDMFY4TVZCSAj64gKF (steps 10–58): same pattern, following the schedule.md card's select(.member_id == <person_id>) advice literally.

Verification

$ env -u GOROOT go build ./...        # clean
$ env -u GOROOT go run ./internal/cmd/skilldoc check
skilldoc: cards OK
$ env -u GOROOT go test ./...
ok  github.com/flashcatcloud/flashduty-cli/cmd/flashduty
ok  github.com/flashcatcloud/flashduty-cli/internal/cli
ok  github.com/flashcatcloud/flashduty-cli/internal/cmd/cligen
ok  github.com/flashcatcloud/flashduty-cli/internal/cmd/skilldoc
ok  github.com/flashcatcloud/flashduty-cli/internal/config
ok  github.com/flashcatcloud/flashduty-cli/internal/output
ok  github.com/flashcatcloud/flashduty-cli/internal/skilldoc
ok  github.com/flashcatcloud/flashduty-cli/internal/timeutil
ok  github.com/flashcatcloud/flashduty-cli/internal/update

Skill-card edits are outside the GENERATED: fences, so skilldoc check stays green without a regen.

Surfaced by /audit-ai-sre-sessions.