Add opt-in control for chat markdown markup visibility

[itkonen]

This PR adds a new option to disable code markup hiding.

With eca-chat-hide-markdown-markup set to nil most markdown content (e.g. headings and lists) are shown without markup but code inline code and blocks blocks show inside backticks.

Personally I prefer this because it reveals the language label of code blocks. Furthermore it makes copy-pasting code easier, because, when markup is hidden, backticks randomly get copied along with code.

AI Summary

Adds eca-chat-hide-markdown-markup, a new chat display option that controls whether markdown markup is hidden in ECA chat buffers.

The default preserves the current behavior:

(setq eca-chat-hide-markdown-markup t)

Users who prefer to see markdown syntax can set:

(setq eca-chat-hide-markdown-markup nil)

This keeps markdown fences, backticks, and other markup visible while retaining the existing markdown rendering setup.

Details

This change:

• adds eca-chat-hide-markdown-markup;
• keeps the default chat rendering unchanged;
• applies the same visibility setting to table helper buffers so table display-width calculations and eca-table-open stay consistent with chat rendering;
• documents the option in the README and changelog;
• adds tests for both the default hidden-markup behavior and the visible-markup option.

Motivation

Some users prefer seeing the original markdown syntax in chat buffers, especially around fenced code blocks and inline code. This option makes that behavior configurable without changing the default presentation for existing users.

Testing

Added focused tests covering:

• default hidden-markup behavior;
• opt-in visible-markup behavior;
• table display-width calculations respecting the option;
• eca-table-open respecting the option in the dedicated table buffer.

Local verification:

• git diff --check origin/master...HEAD
• Editor diagnostics for changed files reported no issues.

[ericdallo]

@itkonen I wonder if we need this, because we already can:

(add-hook 'eca-chat-mode-hook
          (lambda () (remove-from-invisibility-spec 'markdown-markup)))

and
eca-chat-copy-at-point to copy code blocks

[itkonen]

Yes, that works fine and is nearly equivalent.

One minor problem is that tables get misaligned if cells contain visible markup **bold text** or `inline code`. Another is caveat is that the trick is not very easy to discover.

But true, I don't know how many people prefer keeping code blocks visible, so I'm not sure if the extra lines and complexity are worth it.

[ericdallo]

I see, I'm approving this as the code to maintain is considerably simple, so not a big deal from my POV, thanks