Add Linear Webhook Support

[Prateek32177]

We want to add webhook support for Linear in tern. Linear sends events for issues, projects, comments, and more.

Tasks

Webhook Handler

• Implement /src/platforms/linear.ts.
• Normalize payloads into tern’s event format.

Platform Config

• Add platformConfigs.linear with supported events (e.g., issue/create, issue/update, comment/create).

Test Cases

• Add JSON fixtures for 3+ events (issue created, issue updated, comment created).
• Write Jest tests in /tests/linear.test.ts.

Docs

• Update /docs/platforms.md with setup steps.
• Mention manual webhook verification steps. See Linear Webhook Verification Docs

Acceptance Criteria

• Linear handler implemented
• Manual webhook verification supported
• Tests with 3+ event types
• Docs updated

Helpful Links: Linear Webhooks Docs