feat: Add Linear integration alongside Jira

[gedeagas]

Summary

Braid currently only supports Jira (via acli CLI) for issue tracking integration. Adding Linear support would expand coverage for teams that use Linear as their project management tool.

Current State

• Jira integration exists via acli CLI wrapper in src/main/services/jira.ts
• Jira sections are rendered in the right panel (JiraSection)
• IPC channels are wired for Jira operations

Proposal

Add a Linear integration that provides equivalent functionality:

• CLI tool: Use the Linear CLI or Linear's GraphQL API directly
• Service layer: New src/main/services/linear.ts service mirroring the Jira service pattern
• IPC channels: linear:* handlers for CRUD operations on issues
• UI: Linear section in the right panel, similar to JiraSection
• Settings: Configuration page for Linear API key/workspace, option to choose between Jira and Linear (or both)

Key Features

• View and link Linear issues to worktrees/sessions
• Create issues from Braid
• Update issue status (e.g., move to "In Progress" when a session starts)
• Display issue details in the right panel

Notes

• Linear has a well-documented GraphQL API and an official SDK (@linear/sdk)
• Using the SDK directly may be simpler than wrapping a CLI tool
• Should support team/workspace selection during setup

[gedeagas]

Update: Prefer integrating via the Linear CLI (linear) over the SDK approach.

This keeps the pattern consistent with how we integrate Jira (via acli CLI wrapper) - wrapping a CLI tool rather than pulling in an SDK dependency. Benefits:

• Consistent architecture: Both Jira and Linear use the same CLI-wrapper pattern in the service layer
• No additional SDK dependency: Avoids adding @linear/sdk to the bundle
• Auth handled externally: Linear CLI manages its own auth flow, just like acli does for Jira
• Simpler service code: Shell out to linear commands, parse JSON output

The service layer (src/main/services/linear.ts) should mirror the jira.ts pattern - spawn linear CLI commands and parse structured output.

[gedeagas]

cc @asaadam as potential dev and QA for this one.