Split Copilot repo conventions from agent-operational guidance

**Problem description**
`.github/copilot-instructions.md` (added in PR #560) mixes two distinct audiences:

1. Repo-contribution conventions (changelog discipline, cross-platform PS 5.1 / PS 7+, `-ErrorAction 'Ignore'` preference, public/private/template coupling, build/test entry points).
2. Agent-operational guidance ("never pipe `./build.ps1` through `Select-Object -Last N`", "tee output to a log file", "parse NUnit XML / HQRM CliXml for failures").

The second category is agent-specific, already partially duplicated in `.github/skills/validate-changes/SKILL.md`, and is not relevant to contributing to Sampler.

Additional minor issues observed in the same set of files:

- Examples use Windows path separators (`output\validate-test.log`) while the document mandates cross-platform code.
- `.github/instructions/test-writing.instructions.md` hard-codes `$script:moduleName = 'Sampler'` in a blueprint flagged as "must use exactly". This reads as a generic Pester-authoring guide but bakes in a Sampler-specific module name, which will mislead contributors copying it into downstream repos that use Sampler's build framework.

**Verbose logs**
N/A

**How to reproduce**
Read `.github/copilot-instructions.md`, `.github/instructions/test-writing.instructions.md`, and `.github/skills/validate-changes/SKILL.md`.

**Expected behavior**
- Agent-operational content lives only in `.github/skills/validate-changes/SKILL.md`.
- `.github/copilot-instructions.md` focuses on repo-contribution conventions.
- Examples use cross-platform path separators.
- The Pester blueprint either parameterizes the module name or explicitly states it is Sampler-specific.

**Current behavior**
Content is duplicated; Windows paths in examples; hard-coded module name in a blueprint marketed as a general template.

**Suggested solution**
- Move the "Running build/test commands without hanging the agent shell" and "Diagnosing test failures from XML output" sections into `skills/validate-changes/SKILL.md` and remove them from `copilot-instructions.md`.
- Normalize example paths to `output/validate-test.log`.
- Update `test-writing.instructions.md` to use `` placeholder wording, or add an explicit "this blueprint is Sampler-specific" note.

**Operating system the target node is running**
N/A

**PowerShell version and build the target node is running**
N/A

**Module version used**
Sampler `Unreleased` / post-#560.