[Dreaming] Dreaming: curate learning atoms for week of 2026-07-08

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `.github/instructions/tasks.instructions.md`, `.github/instructions/tests.instructions.md`

## Weekly learning-atom curation (2026-07-08 to 2026-07-15)

This PR encodes two small recurring lessons observed in human-authored PRs from the past week.

---

### Change 1 — `.github/instructions/tasks.instructions.md`

**What:** Added a bullet clarifying that changing an existing parameter default value is a behavioral change requiring a ChangeWave.

**Why:** PR #14285 (`Default XML tasks to prohibit DTDs`) received three separate inline reviewer comments (on `XmlPeek.cs`, `XmlPoke.cs`, and `XslTransformation.cs`) all pointing out the same issue: flipping a task parameter default from ignore to prohibit changes behavior for existing builds and must be gated behind a ChangeWave. The existing guidance covered *new* parameters but not *changing existing defaults*.

**Not already covered:** Existing bullet says "New optional parameters must default to preserving existing behavior" — does not address changing an already-established default.

---

### Change 2 — `.github/instructions/tests.instructions.md`

**What:** Added a sentence requiring a unit test for every new contract, guard, or boundary condition.

**Why:** Two separate PRs had reviewer comments about missing boundary tests:
- PR #14290 (`Add opt-in partial evaluation`): reviewer flagged that `ItemDefinitions` at the `Properties` stage was undocumented by tests, risking silent regressions.
- PR #14356 (`Preventing coordinator messages from being filtered`): reviewer noted the new null-BEC + HIGH-importance logger path had no unit test.

**Not already covered:** Existing tests instructions cover helpers, platform conditions, and data-driven patterns — nothing about testing boundary/contract conditions.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [Dreaming (learning atoms curation)](https://github.com/dotnet/msbuild/actions/runs/29411365525) · 44.7 AIC · ⌖ 9.09 AIC · ⊞ 6.7K · [◷](https://github.com/search?q=repo%3Adotnet%2Fmsbuild+%22gh-aw-workflow-id%3A+dreaming.agent%22&type=pullrequests)

---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** pushSignedCommits: refusing unsigned push for branch 'dreaming/learning-atoms-2026-07-15-256762709b214f9c': Signed-commit payload violates file-protection policy (request_review): .github/instructions/tasks.instructions.md, .github/instructions/tests.instructions.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/dotnet/msbuild/actions/runs/29411365525)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 29411365525 -n agent -D /tmp/agent-29411365525

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29411365525/aw-dreaming-learning-atoms-2026-07-15.bundle refs/heads/dreaming/learning-atoms-2026-07-15:refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0
git update-ref refs/heads/dreaming/learning-atoms-2026-07-15-256762709b214f9c refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0
git checkout dreaming/learning-atoms-2026-07-15-256762709b214f9c
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0

# Push the branch to origin
git push origin dreaming/learning-atoms-2026-07-15-256762709b214f9c

# Create the pull request
gh pr create --title '[Dreaming] Dreaming: curate learning atoms for week of 2026-07-08' --base main --head dreaming/learning-atoms-2026-07-15-256762709b214f9c --repo dotnet/msbuild
```

Review `.github/instructions/tasks.instructions.md` and `.github/instructions/tests.instructions.md`, then download and inspect the workflow bundle from run 29411365525. Confirm the two documented learning-atom changes and the protected-file review requirement before pushing the recovered branch and opening the specified pull request.

Written by the indexing model from the issue text.