Add opt-in AI features with API key toggle

[serabi]

Background

@Gustavo-Hilario suggested AI-assisted comment drafting while testing the TUI. This led to a broader discussion about integrating AI features behind an opt-in toggle, keeping the core experience lean for users who don't want or need them.

Proposal

Add an "AI Features" tab in settings with:

• Enable/disable toggle
• API key field (stored in OS keychain via existing keyring.go infrastructure)
• Model selection (default to a cheap, fast model like GPT-5.4 Mini or Claude Haiku 4.5)

Tier 1 features (concrete scope)

Draft comment assistance

In the detail view, press a key to generate a comment draft from the issue context and recent comments. The draft is editable before posting — AI suggests, user decides.

Issue summarization

When viewing an issue with a long comment thread, show a condensed summary at the top of the detail view so you can quickly get up to speed.

Smart launch prompts

When launching Claude in a worktree slot, generate a richer context prompt from the issue description, comments, and related issues — instead of just templating in the title and description.

Future directions (Tier 2)

• Natural language search/filtering ("show me unassigned bugs from last week")
• Suggested state transitions based on activity
• Sprint health summaries

Implementation notes

• Settings infrastructure already supports tabbed forms (model_settings.go)
• Keychain storage already exists for Linear API key (keyring.go)
• Adding a third settings tab for AI config would follow existing patterns