fix(forker): reject unsupported agent launchers

[floze-the-genius]

Summary

• replace substring-based launcher detection with strict, case-insensitive agent prefixes
• preserve model-style Claude and Codex identifiers such as claude-sonnet-4-6, o4-mini, and gpt-5.5
• return an explicit error for unknown agents instead of silently launching Claude
• add regression coverage for false positives such as proto4 and not-gpt

Fixes #13

Tests

• go test -count=1 ./internal/forker
• go test -race -count=1 ./...
• go build ./cmd/looprelay
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2 run ./...

AI disclosure

This change was implemented and tested with assistance from OpenAI Codex.