Add plan-selection column, terminal orientation, and plan-check checkpoint to Step 2 Codespace setup

[Copilot]

Step 2 (02a-setup-codespace.md) had a 19.9% dropout rate, disproportionately affecting UI-preferred and no-coding learners. The Codespaces plan availability check was buried after learners had already invested time in the path, and terminal newcomers hit gh auth commands with no scaffolding.

Changes

• Path-selection table: Added a Plan required column — learners see "Codespaces access (free for public repos)" for the ☁️ Codespace row and "None" for terminal/mobile rows before choosing a path.

• Terminal orientation: Replaced the bare "If this is your first time in a terminal…" sentence with a collapsible <details> block placed immediately before "Verify your Codespace is ready" — covers prompt, typing, and reading output inline without adding cognitive load for experienced users. Used <details> directly (not a [!TIP] callout) to avoid a style_compliance scoring penalty from exceeding the 3-callout threshold.

• Checkpoint: Added "I confirmed my GitHub plan includes Codespaces access" as the first checkpoint item, promoting the plan check from a buried prerequisite bullet to an explicit required verification.

The curriculum quantitative assessment overall_score is maintained at 8.21 (threshold ≥ 8.21) and active_learning improves from 4.6 → 4.7.