Custom agents in `.claude/agents/` don't receive MCP tools when spawned as sub-agents via `task` tool

### Describe the bug

Custom agents defined in `.claude/agents/` with `mcps:` declared in their YAML frontmatter do **not** receive MCP tool connections when spawned as sub-agents via the `task` tool. The agent loads and runs, but MCP tools are absent from its tool inventory.

**Workaround:** Moving agent files to `.copilot/agents/` (with symlinks in `.claude/agents/`) resolves the issue — sub-agents then receive all MCP tools correctly.

This may be related to #2630, but is a distinct issue: #2630 covers MCP propagation failing entirely for sub-agents. This bug is specifically about agent **file location** affecting MCP resolution.

### Affected version

1.0.26

### Steps to reproduce the behavior

1. Create a custom agent in `.claude/agents/my-agent.md` with `mcps:` frontmatter:
```markdown
---
name: my-agent
description: Agent that uses MCP tools
mcps:
my-mcp:
type: local
command: some-mcp-command
args: [mcp, my-server]
---
Agent instructions here.
```

2. Configure `.mcp.json` at repo root with the same MCP servers (they work in the main interactive session).

3. Spawn the agent as a sub-agent via the `task` tool:
```
task(agent_type="my-agent", prompt="List all your available tools")
```

4. **Result:** Agent loads but reports **no MCP tools** — only basic tools (view, edit, grep, powershell, etc.)

5. Move agent to `.copilot/agents/my-agent.md`, symlink `.claude/agents/my-agent.md` → `.copilot/agents/my-agent.md`.

6. Reload session and repeat step 3.

7. **Result:** Agent now reports all MCP tools correctly.

### Expected behavior

Agents in `.claude/agents/` should receive MCP tools from their `mcps:` frontmatter when spawned as sub-agents, the same as agents in `.copilot/agents/`.

### Additional context

- **OS:** Windows 11
- **Shell:** PowerShell 7
- **Copilot CLI version:** 1.0.26
- The main interactive session always has MCP tools regardless of agent location
- Session reload was required after moving files (mid-session moves hit a stale file path cache)