feat: support configurable timezone for timestamp display

[carlos-mora-sm]

Problem

All timestamps stored and displayed by Engram use UTC internally.
Users in non-UTC timezones (e.g. America/Bogota, UTC-5) see timestamps
that don't match their local time, making it harder to correlate memories
with actual work sessions.

Proposed Solution

Add timezone support via an environment variable:

ENGRAM_TIMEZONE=America/Bogota

This would convert all displayed timestamps to the specified timezone
without changing the storage format (keep UTC in DB, display in local tz).

Expected Behavior

With ENGRAM_TIMEZONE=America/Bogota:

• mem_context returns timestamps in COT (UTC-5)
• mem_search results show local time
• engram context CLI output shows local time

Current Behavior

Timestamps always display in UTC regardless of system timezone,
even when the host machine is configured to a different timezone.

Environment

• engram: v1.11.0
• OS: macOS (darwin)
• System timezone: America/Bogota (UTC-5)