fix(install): copilot-cli MCP config key (servers → mcpServers) and Copilot instructions path (#616)

[LNTisNotaTree]

Fixes #616.

Bug 1: MCP server written under the wrong JSON key

Copilot CLI reads MCP server definitions from "mcpServers" in ~/.copilot/mcp-config.json, but the installer wrote "servers" — the entry was silently never parsed.

• The copilot-cli platform now uses "key": "mcpServers".
• Installs also migrate a stale code-review-graph entry that an older release left under "servers" (other servers under that key are untouched; the key itself is dropped once emptied), so upgrading users self-heal on the next install instead of ending up with a duplicate/stale blob.
• uninstall now also checks legacy keys, so it can clean configs written by older releases.

The VS Code copilot platform keeps "servers" — that is correct for .vscode/mcp.json.

Bug 2: instructions file at a path Copilot never loads

Copilot only auto-loads instruction files matching .github/instructions/**/*.instructions.md; the installer wrote .github/code-review-graph.instruction.md (flat directory + singular suffix), which is never read.

• Instructions now go to .github/instructions/code-review-graph.instructions.md for both copilot and copilot-cli.
• On install, our injected section is stripped from the legacy path (the file is deleted when nothing else remains; user-authored content or unrecognized sections are left alone).
• uninstall also removes the section from the legacy path for installs created by older releases.

Verification

End-to-end repro from the issue (isolated HOME, pre-existing azure-style server under mcpServers, stale code-review-graph under servers):

{
  "servers": { "azure-mcp-server": { "command": "azmcp" } },
  "mcpServers": {
    "code-review-graph": { "command": "...", "args": ["...", "serve"], "type": "stdio" }
  }
}

…and .github/instructions/code-review-graph.instructions.md is created while a generated legacy .github/code-review-graph.instruction.md is removed.

New tests cover: the corrected key, legacy-key migration (including dropping the emptied servers object), legacy instruction-file removal, preservation of user content sharing the legacy file, and leaving user-authored files without our marker untouched. tests/test_skills.py, tests/test_uninstall.py, tests/test_cli_install.py all pass (225 tests); the full suite shows no new failures relative to main in my environment.