agentc-llamaindex LlamaIndex version constraint blocks classic ReAct agent usage

**Title:** agentc-llamaindex LlamaIndex version constraint blocks classic ReAct agent usage

---

## Problem

The `agentc-llamaindex` package currently requires:

```toml
llama-index = "^0.13.4"
```

This forces projects to use **LlamaIndex 0.13.x**, which introduced breaking API changes that removed the classic ReAct agent interface.

---

## API Changes in LlamaIndex 0.13.x

* **Removed:** `ReActAgent.from_tools()` method
* **Removed:** `.chat()` method on agents
* **Replaced with:** Workflow-based agent system with different API
* **Lost:** Classic verbose ReAct output format (`Thought: / Action: / Observation:`)

---

## Impact

* Cannot use classic ReAct agent with verbose debugging output
* Existing code using `ReActAgent.from_tools()` and `.chat()` breaks
* Development blocked due to dependency version solving failure

---

## Requested Solution

Relax the LlamaIndex version constraint in `agentc-llamaindex` from:

```toml
llama-index = "^0.13.4"
```

To:

```toml
llama-index = ">=0.12.0,<0.14.0"
```

This would allow projects to choose between:

* **LlamaIndex 0.12.x** → Classic ReAct agent API (verbose Thought/Action/Observation output)
* **LlamaIndex 0.13.x** → New workflow API

---

## Workaround

Currently using a temporary workaround by disabling `agentc-llamaindex` dependency, but this removes AgentC integration benefits.

---

## Environment

* **Python:** 3.12
* **agentc-llamaindex version:** 0.0.0
* **Desired LlamaIndex version:** 0.12.x for classic API compatibility

Start by locating the dependency declaration for the agentc-llamaindex package and compare its current LlamaIndex constraint with the requested range. Verify that dependency resolution permits both 0.12.x and 0.13.x, and confirm that the package remains installable with the supported Python environment.

Written by the indexing model from the issue text.