[FEATURE]: Extract Kilo-specific logic from shared `pr.ts`

[hdcodedev]

Issue: Extract Kilo-specific logic from shared pr command

Related: #9579. That change covered only part of this cleanup.

Problem

The shared file packages/opencode/src/cli/cmd/pr.ts still contains Kilo-specific logic.

Goal

Keep shared pr command behavior generic and move Kilo-only behavior into Kilo paths to reduce upstream merge conflicts.

Scope

• Move Kilo session import handling out of packages/opencode/src/cli/cmd/pr.ts.
• Move Kilo launch handling out of packages/opencode/src/cli/cmd/pr.ts.
• Keep generic GitHub checkout and fork remote setup in packages/opencode/src/cli/cmd/pr.ts.

Target Files

• packages/opencode/src/kilocode/cli/cmd/pr-session.ts
• packages/opencode/src/kilocode/cli/cmd/pr-run.ts
• packages/opencode/test/kilocode/pr-helpers.test.ts

Expected Result

Shared pr command only orchestrates generic flow and calls Kilo helpers where needed, with no user-visible behavior change.