Learn eligible Knight spells from NPC trainers

[adrunkhuman]

Summary

• Let playerbots discover tagged NPC trainer offers at runtime and learn one reachable eligible Knight spell through normal dialogue.
• Preserve supply and cash reserves, verify learned state plus exact payment, and avoid repurchase after restart.
• Document the trainer and telemetry contracts and add focused gameplay coverage.

Closes #90

Verification

• pwsh -NoProfile -File scripts/test-knight-spell-contract.ps1 - passed
• docker compose -f server/compose.yaml config --quiet - passed
• pwsh -NoProfile -File scripts/test-playerbot-gameplay.ps1 -Focused -SpellTraining -FullNavigation -CorpseLoot - passed