feat: support Sourcegraph Amp cli

[Harsh9485]

@hugodutka, let me know if you need any changes. I’m not very familiar with the agentAPI code, so if I messed something up, please let me know.

[hugodutka]

The PR looks good overall, thank you! One test case seems to have a mistake in it though.

[hugodutka]

@Harsh9485 one of the tests is failing - not sure if you have access to CI results here. You should be able to see it locally when you run go test ./... in the repo root.

[Copilot]

Pull Request Overview

This pull request adds support for Sourcegraph Amp CLI as a new agent type in the AgentAPI system. It extends the existing framework to handle Amp agent messages alongside other supported agents like Claude, Goose, Aider, Gemini, and Codex.

Key changes include:

• Addition of AgentTypeAmp constant and integration into the agent type system
• Test data for validating Amp message formatting behavior
• Documentation update to include Sourcegraph Amp in the list of supported agents

Reviewed Changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/msgfmt/msgfmt.go	Adds AgentTypeAmp constant and routing to generic message formatting
cmd/server/server.go	Integrates AgentTypeAmp into server agent type definitions and validation
lib/msgfmt/msgfmt_test.go	Includes AgentTypeAmp in the test suite for message formatting
cmd/server/server_test.go	Adds test case for parsing "amp" agent type
lib/msgfmt/testdata/format/amp/*	Test data files for validating Amp message formatting scenarios
README.md	Updates documentation to include Sourcegraph Amp in supported agents list