docs(memory): add README.md for agent-memory component

[ralf003]

Summary

Add a comprehensive README.md for the agent-memory component.

What This PR Does

• Adds src/agent-memory/README.md (342 lines)
• Covers: project overview, architecture, 19 MCP tools reference, security model, FTS5 BM25 index details, git versioning, configuration, project structure

Verification

• All 19 tool names verified against source code (grep on tools.rs)
• Tool parameters verified against #[tool(param)] annotations
• Security model verified against safe_fs/mod.rs
• FTS5 index schema verified against index/store.rs
• Verified on Aliyun Linux test environment

Impact

• Pure documentation, no code changes
• Follows copilot-shell/README.md structure

[CLAassistant]

All committers have signed the CLA.