Correct and clean up developer setup documentation

[papeh]

Documentation was out of sync with reality (both ahead and behind). Correct documentation and remove redundant documentation.

CI-ready checklist

(https://github.com/sillsdev/FieldWorks/blob/main/.github/commit-guidelines.md).

• As much as possible, the change is unit tested.
• Builds & tests pass locally (or I've run the CI-style build via build.ps1, test.ps1, or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.
• I have considered all comments from an AI code reviewer (such as Devin)

Reading this a year from now -- start here

This PR is a documentation correction and de-duplication pass across Docs/CONTRIBUTING.md and Docs/core-developer-setup.md. There's no code behind it and no research artifact was produced or deleted — the reasoning below is all there is.

Decisions, and why

• CONTRIBUTING.md is the source of truth for any contributor. Where the same instruction (e.g. git rename-limit config, the Visual-Studio-vs-VS-Code switching guidance, the AI-Assisted PR Workflow pointer) applied equally to core and non-core developers, it was kept once in CONTRIBUTING.md and removed from core-developer-setup.md, with a cross-reference left behind instead of silently dropping it.
• core-developer-setup.md keeps only what's actually core-developer-specific: write access / SSH key setup, git identity configuration, release branch tracking, branch naming conventions, and release-manager escalation contacts. None of that is meaningful for a non-core contributor, so it wasn't a duplication candidate.

Evidence

• git diff --name-status origin/main...HEAD -- '*.md' shows exactly the two files touched: Docs/CONTRIBUTING.md, Docs/core-developer-setup.md.
• Every concrete path/script named in the edited sections was checked against the tree and resolves: Setup-Developer-Machine.ps1, Build/Agent/Setup-DefenderExclusions.ps1, Build/Agent/Verify-FwDependencies.ps1, Build/Agent/Setup-InstallerBuild.ps1, Docs/workflows/ai-pr-workflow.md, Docs/visual-studio-setup.md, Docs/installer-build-guide.md, Docs/vscode-stability-profile.md, .github/instructions/build.instructions.md, .github/commit-guidelines.md, Docs/workflows/pull-request-workflow.md, Docs/workflows/release-process.md, DistFiles/Graphite/pl/piglatin.ttf, build.ps1, test.ps1.
• Removed rename-limit git config block was confirmed present verbatim in CONTRIBUTING.md's "Git Configuration Reference" section before deleting it from core-developer-setup.md (it had been stated three times across the two files).

Reviewable

This change is 

Co-authored-by: Claude Sonnet 5 noreply@anthropic.com

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   12m 38s ⏱️ +59s
6 011 tests ±0  5 930 ✅ ±0  81 💤 ±0  0 ❌ ±0 
6 020 runs  ±0  5 939 ✅ ±0  81 💤 ±0  0 ❌ ±0 

Results for commit 01a1132. ± Comparison against base commit 1ab408b.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.58%. Comparing base (1ab408b) to head (01a1132).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1124   +/-   ##
=======================================
  Coverage   38.57%   38.58%           
=======================================
  Files        1514     1514           
  Lines      351014   351014           
  Branches    40355    40355           
=======================================
+ Hits       135418   135432   +14     
+ Misses     186380   186368   -12     
+ Partials    29216    29214    -2     

see 3 files with indirect coverage changes

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