Fix chat headers and make them configurable

[joaopluigi]

Context

Header ## 👤 You was not being displayed for user messages:

Before

After

After with custom headers

This PR also add the option to configure custom headers such as:

require('eca').setup({
  chat = {
    headers = {
      user = "> ",
      assistant = "",
    },
  },
})

[Copilot]

Pull Request Overview

Fixes missing user message headers in the chat interface and adds configuration options for customizing chat headers. The PR resolves an issue where user messages weren't displaying the "## 👤 You" header and introduces the ability to customize both user and assistant headers through configuration.

• Fixed user message header display by ensuring user messages are properly added with headers
• Added configurable chat headers through the config system with sensible defaults
• Updated message rendering logic to use the configurable headers consistently

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lua/eca/sidebar.lua	Updated message handling to display user headers and use configurable headers from config
lua/eca/config.lua	Added chat configuration section with default header settings
docs/configuration.md	Added documentation for the new chat header configuration options

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

[nitpick] This change from Logger.trace to Logger.debug increases log verbosity for what appears to be a common occurrence (empty text). Consider keeping it as trace level to avoid cluttering debug logs.

Suggested change

	Logger.debug("Ignoring empty text response")
	Logger.trace("Ignoring empty text response")

[joaopluigi]

.trace does not exists at Logger

[Copilot]

String comparison using line:sub(1, #assistant_header) could fail if assistant_header is empty (length 0). This would cause line:sub(1, 0) to return an empty string, potentially matching any line incorrectly. Add a guard to check if assistant_header is not empty before the comparison.

Suggested change

	if assistant_header == "" then
	return 0
	end

[joaopluigi]

I believe it should be able to match an empty line 🤔, this is iterating backwards so it is important that we match the first empty line it finds from the end to the beginning

[ericdallo]

Can we have the default as your print of the custom headers? it's what other editors do, and TBH I doubt someone will customize that

[joaopluigi]

Can we have the default as your print of the custom headers? it's what other editors do, and TBH I doubt someone will customize that

I want to test this for some time first before making it the default. The code has some logic to get the last assistant message based on the header, and I want to stress this behavior more in the cases where the assistant header is empty. WDYT?

[ericdallo]

Looks good!