fix: Frontmatter formatter error using hve-core agents in Copilot CLI

### Component

VS Code Extension

### Bug Description

After installing the plugin via `copilot plugin install hve-core@hve-core`, all 18 agent files under `agents/` produce warnings on every Copilot CLI launch:

```
custom agent markdown frontmatter is malformed: missing or malformed YAML frontmatter
```

Each agent file in the installed plugin is a single-line relative path reference instead of the actual agent markdown:

```
../../../.github/agents/hve-core/doc-ops.agent.md
```

The Copilot CLI expects agent `.md` files to begin with valid YAML frontmatter (`---` delimited block with at least a `description` field). The relative path string is not parseable as YAML frontmatter, and does not resolve to anything meaningful from the installed plugin directory (`~/.copilot/installed-plugins/hve-core/`).

All 18 agents are affected (17 hve-core + 1 ado).

## Root Cause

The plugin packaging process stores relative path references in agent files rather than inlining the actual content from `.github/agents/`. Commands, instructions, and skills use the same path-reference pattern and are handled correctly by the CLI, but agents require parseable YAML frontmatter which the path stubs do not provide.

## Proposed Fix

Update the plugin packaging to inline the full agent markdown content (YAML frontmatter + prompt body) from `.github/agents/` into the corresponding `agents/` files in the plugin package, matching how the CLI expects to consume them.

To prevent regressions, consider adding CI tests that validate core CLI workflows

## Affected Files

* `hve-core/agents/doc-ops.md`
* `hve-core/agents/implementation-validator.md`
* `hve-core/agents/memory.md`
* `hve-core/agents/phase-implementor.md`
* `hve-core/agents/plan-validator.md`
* `hve-core/agents/pr-review.md`
* `hve-core/agents/prompt-builder.md`
* `hve-core/agents/prompt-evaluator.md`
* `hve-core/agents/prompt-tester.md`
* `hve-core/agents/prompt-updater.md`
* `hve-core/agents/researcher-subagent.md`
* `hve-core/agents/rpi-agent.md`
* `hve-core/agents/rpi-validator.md`
* `hve-core/agents/task-implementor.md`
* `hve-core/agents/task-planner.md`
* `hve-core/agents/task-researcher.md`
* `hve-core/agents/task-reviewer.md`
* `ado/agents/ado-prd-to-wit.md`

## Workaround

Manually copy the actual agent files from `.github/agents/` in the source repo over the stub files in `~/.copilot/installed-plugins/hve-core/`.

### Expected Behavior

Expected to be able to use the agents in the Github CLI in a similar way to VS Code.

### Steps to Reproduce

Using Copilot CLI version 0.0.4199-0 (current as of 2026-02-26):

```bash
plugin marketplace add microsoft/hve-core
copilot plugin install hve-core@hve-core
````

In Copilot: `/agent`

### Additional Context

_No response_