Add OpenCode installer provider

[SrFlipFlop]

Why

The Burp MCP extension already provides a one-click installer for Claude Desktop, but other agent clients currently require users to manually edit their configuration files.

OpenCode was verified as a working client in the docs PR, and it has a stable JSON configuration format that is straightforward to update automatically. Adding a dedicated installer button reduces setup friction and makes Burp's MCP integration feel more complete for users working outside Claude Desktop.

What changed

• add a new OpenCodeProvider installer
• register an Install to OpenCode button in the MCP installation panel
• create or update ~/.config/opencode/opencode.json
• add or replace the mcp.burp entry with a remote server configuration pointing to the Burp MCP endpoint
• preserve unrelated OpenCode config entries when updating the file
• add tests covering config creation and merge behavior

Scope

This PR only adds automatic installation for OpenCode.

It does not:

• add installers for other clients
• change the MCP server runtime behavior
• modify the existing Claude Desktop installer flow

Related issues

• Builds on the client setup work documented in #83.
• Helps with #46 by making one tested client easier to configure.
• Related to #38, which asks for clearer setup outside the Claude Desktop happy path.
• May help users hitting #42 by reducing manual config mistakes around the Burp endpoint.

Manual verification

• verified that OpenCode works against a local Burp MCP server
• verified installer output writes a valid opencode.json
• verified existing OpenCode config entries are preserved while mcp.burp is updated

Tests

• added focused tests for OpenCode config creation
• added focused tests for OpenCode config merge/replacement
• ran ./gradlew test