🚀 [Feature]: Agent clients follow shared instructions (#60)

Agent clients now route to the repository's shared `AGENTS.md`
instructions, keeping their client-specific files concise and aligned.

## New: Shared agent instruction routes

Claude Code reads `.claude/CLAUDE.md`, and GitHub Copilot reads
`.github/copilot-instructions.md`; each points to the same repository
guidance in `AGENTS.md`.

## Adopting this release

No configuration, code, or invocation changes are required beyond
selecting this release.

## Release impact

| Field | Decision |
| --- | --- |
| Effective decision | `release:minor` — a backward-compatible
agent-client capability. |
| Semantic effect | Minor stable release. |
| Release/base coordinates | Final coordinates are resolved by the
release process. |

---
<details>
<summary>Technical details</summary>

### Consumer change record

| Identifier / surface | Before | After | Applicability / prerequisites
| Consumer action | Verification |
| --- | --- | --- | --- | --- | --- |
| Agent instruction routes | Claude Code and GitHub Copilot had no
repository-specific route files. | Both clients route to the shared
`AGENTS.md` instructions through minimal headed Markdown files. |
Consumers using either client in this repository. | No action required.
| Exact file-content, final-newline, and resolved-target validation
passed. |

### Template baseline

Not applicable. This change updates this template's agent-client route
files directly and does not consume a separate integration template.

### Maintainer evidence

Added only the two client route files. Each has a minimal
client-specific H1 so the existing Markdown lint rules apply without
exception; the route targets resolve to the repository-root `AGENTS.md`
file.

</details>

<details>
<summary>Relevant issues (or links)</summary>

- Resolves #59

### Related work

- N/A

</details>

Author: MariusStorhaug

.claude/CLAUDE.md

@@ -0,0 +1,3 @@
+# Claude
+
+@../AGENTS.md

.github/copilot-instructions.md

@@ -0,0 +1,3 @@
+# Copilot instructions
+
+Follow the instructions in [AGENTS.md](../AGENTS.md).