refactor: remove uuid dependency and simplify database operations in …

[pikann]

Summary

• Removes the github.com/google/uuid dependency (go.mod / go.sum) across the entire plugin — ID generation is now handled by PostgreSQL.
• Drops all client-side UUID generation from branches.go, integration.go, pull_requests.go, and webhook.go; INSERT statements no longer include an id column.
• Switches affected queries from db.Exec to db.Query with RETURNING id (or RETURNING id, created_at) to retrieve the database-generated primary key.
• Simplifies linkPRToTask and handlePREvent: removes the pre-SELECT that checked whether a PR was already cached (replaced by the upsert's RETURNING result).
• github_task_branches and github_task_pr_links inserts also drop their explicit id columns.

Test plan

• Link a repository and confirm the response returns the DB-generated repo_id.
• Link a PR to a task (both via API and via webhook push event) and verify pr_id comes from RETURNING.
• Link a branch to a task and confirm no regression in branch-linking behavior.
• Verify go.mod no longer lists github.com/google/uuid.