refactor: switch config format from YAML to TOML

- Replace gopkg.in/yaml.v3 with github.com/BurntSushi/toml
- Update struct tags from yaml to toml
- Update config tests to use TOML syntax
- Add testdata/config.toml, update Helm configmap template

config.toml

@@ -0,0 +1,24 @@
+[server]
+addr = ":8080"
+
+[git]
+type = "gitea"
+base_url = "https://git.example.com"
+token = "test-token"
+
+[cluster]
+kubeconfig = "/dev/null"
+namespace = "proto-gen"
+job_timeout = 600
+max_concurrent = 2
+
+toolchain_registry = "registry.example.com/proto-gen"
+target_repo_template = "{proto_repo}-gen-{lang}"
+
+[[plugins]]
+language = "go"
+image = "proto-gen-go:v1.34.2"
+
+[[plugins]]
+language = "nodejs"
+image = "proto-gen-ts:v0.9.0"