Add timezone offset resolution to Airport Code Resolver

[telivity-otaip]

Description

Derive utc_offset from timezone name using Intl.DateTimeFormat. Handle DST transitions.

Files

• packages/agents/reference/src/airport-code-resolver/resolver.ts
• packages/agents/reference/src/airport-code-resolver/types.ts

Acceptance criteria

• Output includes utc_offset string (e.g., "+05:30", "-08:00")
• Handles DST (summer vs winter offset for same timezone)
• Uses Intl.DateTimeFormat — no external tz library
• Tests for UTC, positive offsets, negative offsets, DST