docs: document memory settings and project context

[Njengah]

Scope

• Add Milestone 4 guidance for project context, memory, and settings.
• Document common CLAUDE.md, .claude/, rules, local settings, and user settings locations.
• Add an AGENTS.md import example and path-scoped .claude/rules/ example.
• Add /memory to the slash command table and update workflow tips/resources.

Official docs used

• https://code.claude.com/docs/en/claude-directory
• https://code.claude.com/docs/en/memory
• https://code.claude.com/docs/en/settings
• https://code.claude.com/docs/en/debug-your-config

Validation

• git diff --check
• fenced code marker balance check
• reviewed diff against docs/safe-workflows-and-permissions

Review notes

• This is stacked on PR docs: add safe workflow and permissions guide #9 because PR docs: add safe workflow and permissions guide #9 is still open.
• The effective diff for this PR is one README commit: 80cb301.