docs(changelog): complete v2026.8.9-2 release audit

[code-yeongyu]

Summary

• completes the v2026.8.9..origin/main changelog audit across all 31 revisions
• duplicates six user-facing AI provider-cache changes into the coding-agent release surface
• documents capability-gated lazy Anthropic SDK loading for warmPromptCache

Coverage

• revisions audited: 31/31
• non-merge commits: 22
• merge commits: 9
• omissions/extras: 0/0
• released changelog sections modified: none

RED

• coding-agent omitted Kimi cache accounting, Bedrock/Claude cache TTL behavior,
  Moonshot/Kimi cache keys, OpenRouter affinity, and expanded cache-control markers
• AI and coding-agent omitted the lazy Anthropic SDK loading fix

GREEN

• semantic concept audit: no missing AI or coding-agent groups
• node scripts/check-pr-changelog.mjs --base origin/main
• git diff --check
• only packages/ai/CHANGELOG.md and
  packages/coding-agent/CHANGELOG.md changed

Release

Target CalVer: 2026.8.9-2

Plan: .omo/plans/release-2026-08-09-2.md

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Summary by cubic

Completes the v2026.8.9-2 changelog audit and adds the missing provider-cache notes plus the capability-gated Anthropic SDK lazy-load fix for warmPromptCache in packages/ai and packages/coding-agent. Clarifies cache TTLs/accounting, affinity/identity markers for OpenRouter and Moonshot/Kimi, and prompt-cache warm behavior to match what shipped.

^{Written for commit 558f5c0. Summary will update on new commits.}