feat(brief): coordinate input — Skywatch.brief(at: [lat, lon]) (#6)

[jayrav13]

Summary

Adds coordinate input to `Skywatch.brief` alongside the existing airport keyword. For coord queries, the composer:

• finds the nearest reporting station via a bbox METAR query (`Metar#fetch_nearest`)
• uses requested coords for WFO lookup, AFD selection, and adverse-conditions polygon checks
• uses the nearest station's airport ID for TAF, winds aloft, and PIREPs
• attaches a `note` field describing the data source + distance, with a WARNING when the nearest station is more than 25 nm away

CLI auto-detects `LAT,LON` vs airport ID:

```
skywatch brief KCDW
skywatch brief 40.688,-74.174
```

Closes #6. First PR in the v1 sprint to extend Skywatch.brief — PR B (ETD #7) and PR C (route #5) follow.

Test plan

• `Metar#fetch_nearest` specs: closest-by-distance, empty bbox, missing-coord stations, bbox-query shape
• Composer specs: airport+at exclusivity, nearest-station note, far-station warning, no-METAR raise, requested-coord polygon containment
• Brief model: optional `note:` field omitted when nil
• CLI: `LAT,LON` parsing + non-zero exit on bad floats
• Backward compat: existing airport flow + `Skywatch.brief(airport:)` API unchanged
• `bundle exec rspec` — 461 examples, 0 failures
• `bundle exec rubocop` — clean

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