Select recent sightings source (offline JSON vs live API)

[kpb]

Goal

• Make target_checklist work in three modes, in priority order:

Priority

1. If WINGIT_RECENT_JSON is set: load recent sightings from that file
2. Else if WINGIT_EBIRD_TOKEN is set: fetch recent sightings live (and cache)
3. Else: return a friendly error explaining required env vars

Notes

• Keep WINGIT_PERSONAL_JSON required.
• Preserve stdio safety (logs to stderr only).

Acceptance criteria

• Docs updated in README
• Behavior tested (at least one test per mode; integration-style ok)