Replace hardcoded agent type strings with enum

[2witstudios]

Problem

Agent type strings ("claude", "codex", "opencode") are hardcoded as raw string arrays in three separate locations:

• AgentCreationSheet.swift:17 — ["claude", "codex", "opencode"]
• PromptCreationSheet.swift:14 — ["", "claude", "codex", "opencode"]
• AgentsHubView.swift:163 — ["", "claude", "codex", "opencode"]

Adding a new agent type requires finding and updating all three. The two variants (with/without empty string for "Any") add confusion.

Additionally, other stringly-typed patterns exist:

• PR review decisions in PRRowView.swift:72-85 ("APPROVED", "CHANGES_REQUESTED", "REVIEW_REQUIRED")
• Diff status codes in DiffCardView.swift:86-110 ("A", "D", "M", "??")
• Prompt scope wire values checked as prompt.source == "global" in multiple places in AgentsHubView.swift

Suggested approach

1. Create an AgentTypeKind enum with allCases for pickers
2. Create a ReviewDecision enum for PR review decisions
3. Create a DiffStatusCode enum for diff statuses
4. Make SavedPrompt.source a typed enum instead of raw string

Files

• apps/purepoint-macos/purepoint-macos/Views/Agents/AgentCreationSheet.swift
• apps/purepoint-macos/purepoint-macos/Views/Agents/PromptCreationSheet.swift
• apps/purepoint-macos/purepoint-macos/Views/Agents/AgentsHubView.swift
• apps/purepoint-macos/purepoint-macos/Views/Detail/PRRowView.swift
• apps/purepoint-macos/purepoint-macos/Views/Detail/DiffCardView.swift