TODO: Adopt AFDATA Language and Timezone Conventions

This plan applies after AFDATA defines:

• *_bcp47 for BCP-47 language tag fields.
• timezone_utc_offset for fixed UTC timezone offsets.
• null or missing config fields as the documented auto/default form for strict-format fields.

Target Config Shape

Replace the current ui.language and top-level timezone config with a workspace-level section:

{
  "workspace": {
    "language_bcp47": null,
    "timezone_utc_offset": "+08:00"
  }
}

Do not preserve old afmail config compatibility unless explicitly requested.

Config Model

• Add WorkspaceSection to MailConfig with:
  • language_bcp47: Option<String>
  • timezone_utc_offset: Option<String>
• Remove UiSection, UiLanguage, and top-level timezone.
• Validate language_bcp47 as null or a BCP-47-like language tag.
• Validate timezone_utc_offset as null, UTC, or fixed UTC offset.
• Normalize lenient timezone inputs only when writing config, not in raw persisted data.
• Add resolver helpers:
  • resolved_language_bcp47() returns explicit config value or detected/default language.
  • resolved_timezone_offset() returns a FixedOffset from config or current detected offset.
• Prefer pinning timezone_utc_offset at init/setup because it affects UID and message-id date stability.

Behavior Changes

• Use workspace.language_bcp47 as the default language for human-readable workspace text.
• Template rendering should try the configured language first, then fallback to supported built-ins, ending at en-US.
• Keep template fallback internal; do not overwrite the configured language intent just because a template is missing.
• Update AGENTS.md templates and the agent-first-mail skill so agent-generated reason text, case names, archive names, summaries, and notes default to workspace.language_bcp47.
• Put mailbox-specific exceptions in the user-editable AGENTS.md section, not in new config fields.
• Replace direct host-local date paths with config-based fixed offset handling.
• Ensure case/archive UID dates, message-id date components, rendered message times, and fallback dates all use the resolved workspace.timezone_utc_offset.
• Expose workspace.language_bcp47 and workspace.timezone_utc_offset through afmail config get/set.
• Remove ui.language and top-level timezone from config access.

Init and Setup

• Change afmail init to:
  • Write workspace.language_bcp47: null.
  • Detect and write the current fixed workspace.timezone_utc_offset, for example +08:00.
  • Return structured output with detected language/timezone and next-step hints.
• Add or plan afmail setup to:
  • Let a human or agent confirm language and timezone.
  • Normalize timezone input before writing.
  • Keep IMAP/SMTP guided setup separate if the scope gets too large.
• Document that:
  • null means auto/default only where documented.
  • timezone_utc_offset is fixed offset only, not an IANA timezone.
  • language_bcp47 follows the AFDATA *_bcp47 convention.

Tests

• Config:
  • Default config contains workspace.language_bcp47: null.
  • Default/init config pins workspace.timezone_utc_offset.
  • Valid language tags pass; obvious invalid values fail if validation is strict.
  • UTC, +08:00, and -05:00 pass and normalize consistently.
  • Top-level timezone and ui.language are rejected when compatibility is not required.
• Rendering:
  • Exact template language is used when available.
  • Unsupported language falls back to a built-in template language without changing configured language_bcp47.
• Agent/workspace templates:
  • Generated AGENTS.md mentions workspace.language_bcp47 as the default language for agent-written reason/name/summary/notes.
• Time:
  • UID dates and rendered local times use workspace.timezone_utc_offset, not host local timezone.
  • Null timezone resolves predictably in status/doctor, with a warning if not pinned.
• CLI docs/help:
  • Generated help/docs mention the new config keys.
  • Generated help/docs no longer mention ui.language.

Assumptions

• AFDATA has already documented *_bcp47, timezone_utc_offset, and null-as-auto config semantics.
• No backward compatibility is required for existing afmail config files.
• timezone_utc_offset remains fixed-offset only; IANA timezone support is out of scope.
• Fine-grained language controls for reason/case/archive/render are intentionally deferred; one workspace language is the default policy.