Promotes the two commits currently ahead on `develop` to `main`.

Closes #329
Closes #339

## What's promoted
- **#367** - carry the template's "In a derived repo:" upstream-drift
reporting paragraph into `.github/copilot-instructions.md`. This was the
last substantive verbatim-carry gap from a full audit of both
convergence issues against `ptr727/ProjectTemplate`.
- **#366** - record two CI/CD process learnings in `AGENTS.md` (mirror
main-only fixes back to `develop` with a content-diff staleness check;
put `Closes #N` on the promotion PR), converged through the Copilot
review loop.

## Convergence audit (why #329 and #339 are done)
Audited every item in both issues against the current template. This
repo is intentionally NuGet-only, no codegen, no Docker, so most items
were already converged by prior work:
- **#329**: the Copilot review runbook
(`.github/copilot-instructions.md`), the AGENTS.md "PR Review Etiquette"
contract, and `.markdownlint-cli2.jsonc` (byte-identical to the
template) are all present. Only the "In a derived repo:" paragraph was
missing (#367).
- **#339**: `.editorconfig` `[*.cs]` marked .NET-only with matching
per-extension EOL rules; AGENTS.md comments house-rule, markdown
trailing-backslash, and versioning-without-"develop-ahead";
`CODESTYLE.md` (General + .NET, Python dropped); `retention-days: 1` on
the one artifact upload. Docker registry-cache guidance is N/A (no
Docker workflow).

Remaining differences are intentional, repo-accurate divergence
(NuGet-only project names, `CODESTYLE.md` build-requirements text
matching this repo's actual `Directory.Build.props`).

## Not included
Issue **#348** (branch-ruleset drift: extra `creation` rule +
`do_not_enforce_on_create`) is maintainer-gated live GitHub config and
is intentionally left for a maintainer decision (whether the extra
`creation` rule is intended). Surfaced separately, not closed here.

## Diff
`develop` is a clean superset of `main` (4 insertions across two doc
files, no deletions - no `main`-only stragglers).