feat: Optimize LLM request layout for provider caching and add opt-in Anthropic prompt caching with CLI session usage summary.

[vinodvx]

Description

• Structure LLM requests for provider caching: stable system/tools prefix, memory then RAG as labeled messages, sanitized history at call time, and tool resolve order native → MCP → A2A → sub-agents → memory → RAG.
• Add Anthropic-only llm.WithPromptCaching (default off) so cache breakpoints stay optional; OpenAI/Gemini ignore it.
• CLI: show_llm_usage / AGENT_SHOW_LLM_USAGE accumulates session token usage and prints a summary on exit; docs updated for caching and CLI usage.

Test plan

• Unit tests: go test ./internal/runtime/base/... ./pkg/llm/... ./pkg/agent/... ./cmd/...
• Anthropic: enable WithPromptCaching(true), run multi-turn; confirm cached_prompt > 0 when eligible
• CLI: set show_llm_usage: true, run a few turns, exit; confirm session usage summary
• Memory/RAG prefetch still appear as labeled messages (not mutated into system prompt)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / chore

Related issues

Closes #43

Checklist

• I have run make check
• I have run task examples:all
• I have run make tidy if I added or removed dependencies
• Commit messages follow conventional commits (e.g. feat:, fix:, docs:)
• I have added/updated tests for my changes
• Documentation is updated if needed

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Project	Status	Preview	Updated (UTC)
agent-sdk-go	🟢 Ready	View Preview	Jul 14, 2026, 6:06 PM

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[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/llm/anthropic/client.go	68.62%	13 Missing and 3 partials ⚠️
internal/runtime/base/sanitize.go	77.77%	2 Missing and 2 partials ⚠️

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Project	Status	Preview	Updated (UTC)
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