Ensure MCP config files end with a trailing newline

[iitenkida7]

boost:install writes MCP config files (.mcp.json, .cursor/mcp.json, .vscode/mcp.json, etc.) without a trailing newline, so the file ends right after the closing }. As a result:

• GitHub shows the \ No newline at end of file marker in diffs whenever the config file is committed.
• Editors and formatters that enforce a final newline (EditorConfig's insert_final_newline, Prettier, etc.) re-add it, producing a noisy one-line diff after every boost:install run.

This is the same class of issue previously fixed for guideline files in #113 and for skill files in #790 — Install\Mcp\FileWriter was the remaining writer without the guard.

Changes

• FileWriter::writeFile() now ensures the written content ends with a newline, reusing the exact ensureTrailingNewline() implementation from SkillWriter (Fix path separators and trailing newlines in install output #790). Since every write path (new file creation, plain JSON update, JSON5 in-place update) goes through writeFile(), all generated configs are covered.
• The newline is only appended when missing, so user-edited JSON5 files that already end with a newline are left untouched — no double newline.
• Added tests for all three write paths and updated the exact-match expectation in AgentTest accordingly.

There is no behavioral change beyond the final byte of the written file — existing configs are parsed and re-written exactly as before.