docs: add 1000-line customization guide covering all user-configurable systems

New docs/customization.md covers:
- Custom Agents: full YAML schema, override builtins, tool access, LLM overrides, complete examples
- Custom Pipelines: phase ordering, groups, when modes, execute loop, slots, Rust example
- Quality Blocks: detection, lint/test/build commands, safe prefixes, tester hints
- Language Packs: composition, apply flow, what happens on apply
- Custom Stacks: full schema with provider, performance, quality, harness settings
- Skills (SKILL.md): frontmatter schema, resolution algorithm, creation commands
- Project Memory: all .slmcode/ files explained with complete examples
- Config Reference: every config.yaml field documented

Updated mkdocs.yml to include customization page in navigation.

Author: piotrlaczkowski