Generalize CodexAgentBackend → LLMAgentBackend (#234)

[larstalian]

Summary

CodexAgentBackend (src/openrange/agent_backend.py) had no codex-specific code — it just wraps an LLMBackend into a single-shot, tool-less AgentBackend, defaulting to CodexBackend when no backend is passed. The "Codex" in the name was misleading, since the only codex-specific code lives in CodexBackend (src/openrange/llm.py).

This renames the class to LLMAgentBackend so "Codex" appears in exactly one place.

• Clean rename, no backward-compat alias (repo is pre-1.0; an alias would defeat the point of Generalize CodexAgentBackend — it's not actually codex-specific #234).
• The CodexBackend default-fallback behavior is kept intact — the constructor still defaults to CodexBackend(model=model) when no backend is given. Only the error strings and docstrings were updated to say LLMAgentBackend.
• Updated every reference: agent_backend.py, __init__.py (import + __all__), tests/test_agent_backend.py, tests/test_office_persona.py (including the tool-injection error-message assertions), examples/codex_eval.py, packages/openrange-pack-sdk/_protocols.py, and packages/openrange-pack-sdk/README.md.
• Re-grepped CodexAgentBackend across the repo: zero hits remaining.

Closes #234

Test plan

• ruff check on impacted files — clean
• mypy on src/openrange/agent_backend.py + src/openrange/__init__.py — clean
• pytest tests/test_agent_backend.py tests/test_office_persona.py — 29 passed, 2 skipped (strands optional dep)

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