feat: add --tz flag to agenda and mail commands

[corruptmem]

Closes #7

Changes

• agenda: new --tz <IANA timezone> flag converts start.dateTime and end.dateTime to the given timezone with explicit numeric offset (e.g. 2026-04-04T10:00:00+01:00)
• mail: new --tz flag converts receivedDateTime to the given timezone
• Default is UTC — no breaking change to existing callers
• Invalid timezone names return a clear error (unknown timezone "Bad/Zone": ...)

Implementation notes

• Uses Go stdlib time.LoadLocation — no new dependencies
• Handles Graph's truncated datetime strings (no trailing Z) via multiple parse layouts
• Conversion applied after client-side filters, before emit
• Both JSON and text output formats benefit automatically

[corruptmem]

[Judgy Review Agent]

Overall: this gets the job done but there are some gaps worth addressing.

Correctness issues:

1. UTC short-circuit is wrong. convertTZ checks loc == time.UTC (pointer comparison) but time.LoadLocation("UTC") returns the same pointer as time.UTC, so this is fine — however convertMailTZ and convertAgendaTZ also skip when loc == time.UTC, which means the existing Z-terminated Graph datetimes are left as-is (e.g. 2026-04-03T10:00:00Z). That's fine for UTC, but the logic is a bit implicit and fragile.

2. Graph agenda datetimes have no timezone suffix. The Graph calendar API returns datetimes in the event's timezone (e.g. 2026-04-03T10:00:00.0000000) with a separate timeZone field. Parsing these as UTC (the zero time.Parse behaviour for no-offset strings) is technically wrong — the time will be off if the event was created in a non-UTC calendar timezone. This is a pre-existing issue but worth noting since the new flag will surface it.

3. No tests for the new helpers. parseLocation, convertTZ, convertMailTZ, convertAgendaTZ are all untested. Given the subtlety of the Graph datetime format, even a few table-driven unit tests would catch regressions.

Minor nits:

• The variable name agendaLoc/agendaErr in cmdAgenda is inconsistent with the loc/err pattern used in cmdMail.
• The --tz help text says "e.g. Europe/London" for agenda and "e.g. America/New_York" for mail — pick one and be consistent.

The core feature works and the approach (stdlib only, default UTC, no-op short-circuit) is sound. Address the variable naming, add tests, and document the Graph timezone caveat in the help text.