refactor: remove interval-based (20th message) cache control

[lewis617]

Summary

Removes the interval-based (20th message) cache control logic to simplify the cache strategy.

Changes

Code (cacheControlUtils.ts):

• Removed findIntervalMessageIndex function
• Removed addCacheControlToLastToolCall function (only used by interval logic)
• Removed interval block from transformMessagesForClaudeCache
• Kept: system message caching, tool definition caching

Tests (aiService.cacheControl.test.ts):

• Removed 14 interval-related tests
• 45 tests passing

Spec (spec.md):

• Removed User Story 2 (Interval-Based Message Cache)
• Removed FR-003 through FR-006, FR-011
• Renumbered remaining FRs (001-006)

Spec Companion Files:

• Updated plan.md, research.md, data-model.md, tasks.md, quickstart.md, cache-control-api.md

Remaining Cache Control

1. System message — first system message gets cache_control: { type: "ephemeral" }
2. Tool definitions — last tool definition gets cache control

Testing

All 3078 agent-sdk tests pass.