chore: rewrite code comments that cite old plan codes

[MendesMat]

Closes #39

What

Rewrites the code comments that cite plan codes of closed issues (T12, D3, D3 of #7, plan of #7) so they explain themselves, and puts three sentences in the singular now that only one container is left (AppHost.cs, local-environment.md). Also updates one sentence of testing.md that cited "D5". Only comments and the two guides change.

Why

The coding conventions say a comment explains itself and cites at most business rule IDs. The plan codes pointed to nothing in the repository, and T5 looked like the test IDs of the new flow (#34-T05).

Tests

No numbered tests: the issue does not change behavior. It is covered by the acceptance searches below and by the whole suite (243 tests before and after, all green).

How to test

git grep -nE "\b(T[0-9]{1,2}|D[0-9]{1,2})\b" -- '*.cs'   # no output
git grep -nE "#[0-9]+" -- '*.cs'                           # no output
git diff origin/main -- '*.cs'                             # comment lines only
powershell.exe -NoProfile -File check.ps1                  # in backend/ControlService

Checklist

• Developed test-first: not applicable, only comments change; the suite stays at 243 green tests
• check.ps1 passes locally: build without warnings and all tests
• Nothing beyond the issue: no package, abstraction, migration or contract change
• Documentation updated (docs/, README) when behavior or decisions changed
• User-facing messages are verbatim from the feature document (CNV-16): no message changed