feat: replace static API token with per-request Gitea PAT pass-through

Callers now supply their own Gitea PAT as a Bearer token; the server validates
it against GET /api/v1/user and threads it through context to all downstream
Gitea API calls. GITEA_API_TOKEN env var and the GiteaAPIToken config field are
removed.

internal/config/config.go

@@ -8,7 +8,6 @@ import (
 type Config struct {
 	Port            string   // GITEA_MCP_PORT, default 8080
 	GiteaBaseURL    string   // GITEA_BASE_URL, e.g. https://gitea.d-ma.be
-	GiteaAPIToken   string   // GITEA_API_TOKEN — bot user token
 	AllowedOwners   []string // GITEA_MCP_ALLOWED_OWNERS, comma-separated, default "mathias"
 	OriginAllowlist []string // GITEA_MCP_ORIGIN_ALLOWLIST, comma-separated
 }
@@ -17,7 +16,6 @@ func Load() (Config, error) {
 	cfg := Config{
 		Port:            envOr("GITEA_MCP_PORT", "8080"),
 		GiteaBaseURL:    os.Getenv("GITEA_BASE_URL"),
-		GiteaAPIToken:   os.Getenv("GITEA_API_TOKEN"),
 		AllowedOwners:   splitCSV(envOr("GITEA_MCP_ALLOWED_OWNERS", "mathias")),
 		OriginAllowlist: splitCSV(os.Getenv("GITEA_MCP_ORIGIN_ALLOWLIST")),
 	}