docs: avoid root todo filename case collision

[adliebe]

Summary

Fixes #257.

This removes the root-level TODO.md / todo.md case collision that makes a normal Windows checkout warn during clone and appear dirty before any edits are made.

Changes

• Keep root TODO.md as the canonical current roadmap.
• Move the older lowercase todo.md checklist to docs/legacy-project-todo.md.
• Update the docs tree in docs/tech-stack.md so it no longer points at a lowercase root todo.md.

Validation

git diff --check HEAD~1..HEAD
case_duplicate_count 0

I also reproduced the original issue on a normal Windows checkout, where Git warned that TODO.md and todo.md collided and git status --short showed M TODO.md immediately after clone.

npx prettier@3.7.4 --check docs/tech-stack.md TODO.md docs/legacy-project-todo.md still reports existing markdown formatting differences in those docs, so I left formatting untouched to keep this PR focused on the cross-platform checkout bug.

[adliebe]

Submitted for the active Ugig request to test the repo, file a bug, and submit a PR fix: https://ugig.net/gigs/abd6b2a0-e728-48cf-a46f-f99e419ed94e

Bug report: #257
Fix PR: #258

The issue is reproducible on a normal Windows checkout because the repo tracks both TODO.md and todo.md, which collide on case-insensitive filesystems.

[greptile-apps]

Greptile Summary

This PR resolves a cross-platform filename case collision between root-level todo.md and TODO.md that caused Git warnings on Windows checkouts and an immediately-dirty working tree after clone.

• todo.md is renamed to docs/legacy-project-todo.md with no content changes, preserving the older checklist under the docs directory.
• docs/tech-stack.md is updated to reflect the new file location — todo.md becomes TODO.md at the root, and legacy-project-todo.md is correctly nested under the docs/ branch in the directory tree.

Confidence Score: 5/5

Documentation-only rename with no logic, code, or configuration changes — safe to merge.

The change is a straightforward file rename and a corresponding tree-listing update in one docs file. No application code, configuration, or data is touched, and the tree notation in docs/tech-stack.md correctly nests legacy-project-todo.md under docs/. No issues were found.

No files require special attention.

Important Files Changed

Filename	Overview
docs/legacy-project-todo.md	Renamed from root todo.md to docs/legacy-project-todo.md to eliminate the Windows case-collision with TODO.md; content is unchanged.
docs/tech-stack.md	Directory tree updated: todo.md reference replaced with TODO.md, and legacy-project-todo.md correctly nested under the existing docs/ branch using standard tree notation.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Before"] --> B["todo.md (root)"]
    A --> C["TODO.md (root)"]
    B -. "case collision on Windows" .-> C

    D["After"] --> E["TODO.md (root — canonical roadmap)"]
    D --> F["docs/"]
    F --> G["legacy-project-todo.md"]

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