docs: rewrite customization guide to document schema commands (#582)

* docs: rewrite customization guide to document schema commands

The old guide described a painful manual process for schema customization
(mkdir, npm list, cp commands) and even listed "No scaffolding" as a
limitation. But the `openspec schema` commands have existed for a while:

- `schema fork` - copy existing schema to customize
- `schema init` - create new schema from scratch
- `schema validate` - check schema structure
- `schema which` - debug resolution precedence

Rewrote the guide to:
- Lead with the actual CLI commands instead of manual steps
- Remove the misleading "Current Limitations" section
- Add practical examples (TDD workflow, adding review artifact)
- Structure progressively: config → custom schemas → global overrides

* docs: add language tags to code blocks in customization guide

Address review feedback from CodeRabbit:
- Add 'text' language tag to directory tree code blocks
- Satisfies MD040 markdown lint rule

Author: TabishB

docs/cli.md

@@ -745,7 +745,7 @@ spec-driven resolves from: package
 **Schema precedence:**
 
 1. Project: `openspec/schemas/<name>/`
-2. User: `~/.openspec/schemas/<name>/`
+2. User: `~/.local/share/openspec/schemas/<name>/`
 3. Package: Built-in schemas
 
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