v0.2.0: Compact context-planning responses

MCP now returns one minified JSON payload containing only status, used tokens, window size, remaining tokens, and log event age. This reduces repeated context overhead and prevents newer MCP SDKs from duplicating the response in structuredContent.

This changes the MCP response format: last_request.total_tokens becomes used_tokens, and remaining_estimate.tokens becomes remaining_tokens. Tool arguments, CLI output, and the Python library remain compatible. An explicitly supplied compaction threshold adds compaction_remaining_tokens; otherwise the field is omitted.

Unknown, stale, and post-compaction states remain explicit. Errors retain actionable codes and messages. Full timestamps, component counters, and other diagnostics remain available through the CLI.

Validation: 91 synthetic tests; MCP stdio checks with SDK 1.9.2 and 1.29.1; Windows, macOS, and Linux CI. Assets include a wheel and source archive.

Restart an existing context-meter MCP server or its host after upgrading so it loads the new code and tool description.