fix(md): require 4+ spaces for indented code blocks (CommonMark)

[kiwigitops]

Fixes #1388.

Problem

transformMarkdown() in src/md.ts treats any line indented with 2 or more spaces (after a blank line) as a markdown indented code block — i.e. raw JS to execute. CommonMark requires 4 or more spaces for an indented code block.

The 2-space cutoff incorrectly captures list-item continuation lines, which are conventionally indented 2 spaces to align with the content after - . Those continuation lines are prose, not code, but the parser hands them to the JS engine as raw code and the script blows up with SyntaxError: Unexpected identifier '<word>'.

The reporter's case is a README with prose continuation under a list item followed by a fenced shell block. The prose line begins with two spaces, so the old regex treated it as executable code.

Fix

-  const tabRe = /^(  +|\t)/
+  const tabRe = /^( {4,}|\t)/

Aligns the indented-code-block threshold with CommonMark: 4+ spaces or 1 tab. Tab-indented blocks keep working unchanged.

Test plan

• Updated the existing tab-indented block test to assert the new CommonMark-compliant 4-space behavior.
• Added a regression test for 2-space indented list-item continuation prose from [Bug]: cannot parse indented content #1388.
• Existing fenced-code-block and plain-line transform tests still pass.

[antonmedv]

@antongolub LGTM

[antongolub]

I'd prefer more explicit form: /^( {4,}|\t)/

[kiwigitops]

Updated in 5b2f8e5 to use the explicit regex form you suggested:

/^( {4,}|\t)/