docs: add organizational extension strategy to README

[awsjosh]

Summary

Adds organizational extension strategy guidance (two-layer approach and distribution model) to the Extensions section of the README.

Changes

• Added a new "Organizational Complexity" subsection under Extensions in README.md
• Covers Layer 1 (Org-Wide Extensions) and Layer 2 (Team-Specific Extensions) with directory structure examples
• Includes a Distribution Approach section with a lifecycle event table covering AWS updates, org rule changes, team onboarding, and team-specific additions
• Content sourced from the AI-DLC Customer Extensions guide (sections 4 and 6)

User experience

Before: The Extensions section explains how extensions work, lists built-in extensions, and describes how to add your own. There is no guidance on managing extensions across multiple teams or organizations.

After: The Extensions section now includes an "Organizational Complexity" subsection that provides a clear two-layer strategy for org-wide and team-specific extensions, along with a practical distribution approach using git subtree. Organizations can follow this guidance to scale AI-DLC extensions without merge conflicts or modifying shipped files.

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Test Plan

• Open the README.md and verify the "Organizational Complexity" subsection appears under Extensions, after "Adding Your Own Extensions" and before "Tenets"
• Confirm heading levels are consistent: ### for the subsection, #### for sub-parts
• Verify the two directory structure code blocks render correctly
• Verify the distribution lifecycle table renders correctly
• Confirm no existing content was modified — the addition is purely additive

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[awsjosh]

@Kalindi-Dev - I have updated with your feedback from the previous PR. Thanks!

[Kalindi-Dev]

Thanks for addressing the feedback from #224 — the naming consistency, table references, and subtree rationale all look much better.

A couple of follow-up observations after looking at this more closely:

1. Framing subtree alongside the existing distribution model

The repo currently distributes AI-DLC via zip files on GitHub Releases, and the README setup section describes that flow. Since subtree is being introduced here as the recommended org-level approach, it might help to briefly acknowledge the
relationship — e.g., "For individual developers, downloading a release zip is sufficient. For organizations managing extensions across multiple teams, git subtree offers tighter integration." That way readers don't wonder which approach is
canonical. This would also need a broader team review since it introduces git subtree as a recommended org-level approach — we should align on whether that's the direction we want to endorse in the docs.

2. Install script note — clarification needed

Upon looking more closely, the Note block references "the 'install script' referenced in some AI-DLC distribution guides". These guides don't appear to exist in the repo or linked from it, and no install script exists in the codebase. Can you clarify the exact install script this is referring to? Apologies if I missed pointing this out in the previous review.

3. Git subtree link

The linked reference (Git-Tools-Advanced-Merging#_subtree_merge) covers the subtree merge strategy, which is a different concept from the git subtree command. A link to the git-subtree
documentation or a practical guide would be more useful here.

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Minor housekeeping: should #224 be closed now that this supersedes it?

[awsjosh]

Updating based on feedback, will create a new PR

[Kalindi-Dev]

@awsjosh put in my updated comments, please have a look. Looking into this more deeply, so have modified my comments

[awsjosh]

Sure thing, thanks @Kalindi-Dev - I have incorporated the latest feedback and created PR #245