docs: update imports and install commands to use cua metapackage (#1228)

* docs: update imports and install commands to use cua metapackage

- Replace `from cua_sandbox import` / `from agent import` / `from agent.tools import` / `from agent.callbacks import` with `from cua import`
- Replace `pip install cua-sandbox` / `pip install cua-agent[...]` with `pip install cua[...]`
- Replace all-caps CUA (brand name) with Cua in prose (preserving env vars and CUA-Bench)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs(readme): add pip install cua snippet, update Python version, fix imports

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(docs): revert versioned cua-agent==X.Y.Z references back to cua-agent

Those versions were published as cua-agent, not cua.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(cua): expose cua_sandbox.runtime classes from cua metapackage

Allows `from cua import QEMURuntime, TartRuntime` etc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(cua): add cua.runtime and cua.tools submodules; fix docs imports

- cua.runtime re-exports all cua_sandbox.runtime classes (QEMURuntime, TartRuntime, etc.)
- cua.tools re-exports agent.tools + ToolError/IllegalArgumentError from agent.types
- Remove runtime symbols from cua top-level __init__
- Fix docs: from cua import BrowserTool/BaseTool/ToolError → from cua.tools import ...
- Fix docs: from cua import TartRuntime/QEMURuntime → from cua.runtime import ...

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(cua): add cua.callbacks submodule; fix callbacks.mdx imports

- cua.callbacks re-exports all agent.callbacks handlers
- Fix docs/cua/guide/fundamentals/callbacks.mdx to use from cua.callbacks import ...

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: replace pip install "cua[all]" with pip install cua

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs(examples): replace cua-agent/cua-sandbox in requirements.txt blocks with cua

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: replace 'import cua_sandbox as cua' with 'import cua'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: consolidate consecutive 'from cua import' lines into single imports

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>