Improve commit line break unwrapping for variable-length list markers

[Copilot]

The unwrapCommitMessageBody function assumed fixed 2-space continuation indent for all list items. This broke unwrapping for numbered lists with multi-digit markers (10., 100.) and didn't match GitHub's lenient continuation rules.

Changes

• Track content indent instead of marker indent - List continuations now compute the actual text start position (1. = 3, 10. = 4, etc.)
• Allow 1+ space continuations - GitHub accepts any indentation ≥1 space for list continuations (still treating 4+ as code blocks)
• Preserve list context across blank lines - Multi-paragraph lists now unwrap correctly without losing the list item association
• Trim whitespace in plain text joining - Prevents double spaces when joining lines with leading whitespace

Example

Before (broken):

10.  Multi-digit numbers should also
     work for a continuation.

→ Lines not joined (treated as separate due to hardcoded +2 indent check)

After (fixed):

10.  Multi-digit numbers should also work for a continuation.

Test coverage added

• Multi-digit numbered lists
• Multi-paragraph lists (numbered and asterisk)
• Code blocks within list items
• Variable space continuations (1-3 spaces)
• Nested versions of all above

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• update.code.visualstudio.com
  • Triggering command: /opt/hostedtoolcache/node/20.20.0/x64/bin/node /opt/hostedtoolcache/node/20.20.0/x64/bin/node ./out/src/test/runTests.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve commit line break unwrapping</issue_title>
<issue_description>> Opened #8335 to address this.

Thanks @alexr00 for improving this! Unfortunately I think the follow-up PR might have some problems too, and may need some more tests. See #8335 (comment)

Originally posted by @justingrant in #8255 (comment)
</issue_description>

<agent_instructions>Pay special attention to #8335 (comment)</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve commit line break unwrapping #8345

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